fix(dedup): wire merge advisor JSON schema + flexible field_merges parser

Gemini returned field_merges as an array without structure constraint,
causing json.Unmarshal to fail with "cannot unmarshal array into Go struct
field of type map[string]mergeFieldDecision".

- Pass merge_advisor_schema.json via JSONSchema instead of bare JSONMode
- Add parseFieldMerges() that accepts both object and array LLM formats
- Validate target_id is one of the two input market IDs after parsing
- Fix schemaFromMap: minimum/maximum are supported by genai.Schema v1.54

backend/internal/domain/market/merge_advisor.go

@@ -17,6 +17,9 @@ import (
 //go:embed assets/merge_advisor_prompt.de.md
 var mergeAdvisorPromptDE string
 
+//go:embed assets/merge_advisor_schema.json
+var mergeAdvisorSchemaJSON []byte
+
 // ErrNotDuplicate is returned by MergeAdvisor.Propose when the similarity
 // verdict confidently says the two markets are not the same.
 var ErrNotDuplicate = errors.New("markets are not duplicates")
@@ -41,12 +44,12 @@ type mergeFieldDecision struct {
 
 // mergeAdvisorResponse is the raw JSON shape the LLM returns.
 type mergeAdvisorResponse struct {
-	TargetID     string                        `json:"target_id"`
-	TargetReason string                        `json:"target_reason"`
-	FieldMerges  map[string]mergeFieldDecision `json:"field_merges"`
-	Flags        []string                      `json:"flags"`
-	Confidence   float64                       `json:"confidence"`
-	Summary      string                        `json:"summary"`
+	TargetID     string          `json:"target_id"`
+	TargetReason string          `json:"target_reason"`
+	FieldMerges  json.RawMessage `json:"field_merges"`
+	Flags        []string        `json:"flags"`
+	Confidence   float64         `json:"confidence"`
+	Summary      string          `json:"summary"`
 }
 
 // MergeAdvisor calls the LLM to propose how to merge two market editions.
@@ -74,7 +77,7 @@ func (a *MergeAdvisor) Propose(ctx context.Context, mA, mB Market, verdict enric
 	resp, err := a.ai.Chat(ctx, &ai.ChatRequest{
 		SystemPrompt:  mergeAdvisorPromptDE,
 		UserMessage:   userMsg,
-		JSONMode:      true,
+		JSONSchema:    json.RawMessage(mergeAdvisorSchemaJSON),
 		Grounded:      false,
 		Temperature:   0.1,
 		CallType:      "merge_advisor",
@@ -93,6 +96,14 @@ func (a *MergeAdvisor) Propose(ctx context.Context, mA, mB Market, verdict enric
 	if err != nil {
 		return MarketMergeProposal{}, fmt.Errorf("invalid target_id %q: %w", raw.TargetID, err)
 	}
+	if targetID != mA.ID && targetID != mB.ID {
+		return MarketMergeProposal{}, fmt.Errorf("target_id %q is not one of the two input markets", raw.TargetID)
+	}
+
+	fieldMerges, err := parseFieldMerges(raw.FieldMerges)
+	if err != nil {
+		return MarketMergeProposal{}, fmt.Errorf("parse field_merges: %w", err)
+	}
 
 	conf := raw.Confidence
 	if conf < 0 {
@@ -110,7 +121,7 @@ func (a *MergeAdvisor) Propose(ctx context.Context, mA, mB Market, verdict enric
 	return MarketMergeProposal{
 		TargetID:     targetID,
 		TargetReason: raw.TargetReason,
-		FieldMerges:  raw.FieldMerges,
+		FieldMerges:  fieldMerges,
 		Flags:        flags,
 		Confidence:   conf,
 		Summary:      raw.Summary,
@@ -118,6 +129,40 @@ func (a *MergeAdvisor) Propose(ctx context.Context, mA, mB Market, verdict enric
 	}, nil
 }
 
+// parseFieldMerges handles two LLM output formats for field_merges:
+//   - object: {"name": {"source":"a","value":"...","reason":"..."}, ...}
+//   - array:  [{"field":"name","source":"a","value":"...","reason":"..."}, ...]
+func parseFieldMerges(raw json.RawMessage) (map[string]mergeFieldDecision, error) {
+	if len(raw) == 0 {
+		return map[string]mergeFieldDecision{}, nil
+	}
+	var m map[string]mergeFieldDecision
+	if err := json.Unmarshal(raw, &m); err == nil {
+		return m, nil
+	}
+	var arr []struct {
+		Field  string          `json:"field"`
+		Source string          `json:"source"`
+		Value  json.RawMessage `json:"value"`
+		Reason string          `json:"reason"`
+	}
+	if err := json.Unmarshal(raw, &arr); err != nil {
+		return nil, fmt.Errorf("cannot parse as object or array: %w", err)
+	}
+	m = make(map[string]mergeFieldDecision, len(arr))
+	for _, e := range arr {
+		if e.Field == "" {
+			continue
+		}
+		var val any
+		if len(e.Value) > 0 {
+			_ = json.Unmarshal(e.Value, &val)
+		}
+		m[e.Field] = mergeFieldDecision{Source: e.Source, Value: val, Reason: e.Reason}
+	}
+	return m, nil
+}
+
 type advisorMarketInput struct {
 	ID            string `json:"id"`
 	Name          string `json:"name"`

backend/internal/domain/market/merge_advisor_test.go

@@ -29,15 +29,16 @@ func (p *stubAIProvider) Chat(_ context.Context, _ *ai.ChatRequest) (*ai.ChatRes
 }
 
 func validAdvisorResponse(targetID string) string {
+	fm, _ := json.Marshal(map[string]mergeFieldDecision{
+		"name": {Source: "a", Value: "Testmarkt Berlin", Reason: "established name"},
+	})
 	resp := mergeAdvisorResponse{
 		TargetID:     targetID,
 		TargetReason: "Market A has more complete data",
-		FieldMerges: map[string]mergeFieldDecision{
-			"name": {Source: "a", Value: "Testmarkt Berlin", Reason: "established name"},
-		},
-		Flags:      []string{},
-		Confidence: 0.9,
-		Summary:    "Beide Eintraege bezeichnen denselben Markt. Daten von A werden uebernommen.",
+		FieldMerges:  json.RawMessage(fm),
+		Flags:        []string{},
+		Confidence:   0.9,
+		Summary:      "Beide Eintraege bezeichnen denselben Markt. Daten von A werden uebernommen.",
 	}
 	b, _ := json.Marshal(resp)
 	return string(b)
@@ -118,6 +119,38 @@ func TestMergeAdvisor_ProviderErrorPropagates(t *testing.T) {
 	}
 }
 
+func TestMergeAdvisor_ParsesArrayFieldMerges(t *testing.T) {
+	a := Market{Name: "Markt A", City: "Berlin"}
+	b := Market{Name: "Markt B", City: "Berlin"}
+	// LLM returns field_merges as array (Gemini's unguided default format)
+	response := `{
+		"target_id": "` + a.ID.String() + `",
+		"target_reason": "More complete",
+		"field_merges": [
+			{"field": "name", "source": "a", "value": "Testmarkt", "reason": "shorter"},
+			{"field": "stadt", "source": "b", "value": "Hamburg", "reason": "correct"}
+		],
+		"flags": [],
+		"confidence": 0.85,
+		"summary": "Same market."
+	}`
+	p := &stubAIProvider{response: response}
+	v := enrich.Verdict{Same: true, Confidence: 0.9}
+
+	advisor := NewMergeAdvisor(p)
+	proposal, err := advisor.Propose(context.Background(), a, b, v)
+
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(proposal.FieldMerges) != 2 {
+		t.Errorf("expected 2 field merges, got %d", len(proposal.FieldMerges))
+	}
+	if proposal.FieldMerges["name"].Source != "a" {
+		t.Errorf("expected name.source=a, got %q", proposal.FieldMerges["name"].Source)
+	}
+}
+
 func TestMergeAdvisor_InvalidJSONErrors(t *testing.T) {
 	p := &stubAIProvider{response: "not json at all"}
 	a := Market{Name: "Markt A"}

backend/internal/pkg/ai/gemini.go

@@ -346,7 +346,7 @@ func promptHashShort(system, user string) string {
 func schemaFromMap(m map[string]any) *genai.Schema {
 	// Keys that genai.Schema does not support — log a warning so the workaround
 	// (researcher_schema_simple.json) stays visible rather than silently disappearing.
-	unsupported := []string{"pattern", "minLength", "maxLength", "minimum", "maximum",
+	unsupported := []string{"pattern", "minLength", "maxLength",
 		"additionalProperties", "$ref", "$defs", "if", "then", "else"}
 	var dropped []string
 	for _, k := range unsupported {
@@ -398,5 +398,11 @@ func schemaFromMap(m map[string]any) *genai.Schema {
 			}
 		}
 	}
+	if minVal, ok := m["minimum"].(float64); ok {
+		s.Minimum = &minVal
+	}
+	if maxVal, ok := m["maximum"].(float64); ok {
+		s.Maximum = &maxVal
+	}
 	return s
 }