feat(tools): graceful truncation for fetch_url with LLM retry hints

- Remove curl --max-filesize to avoid hard failures on large pages
- Add Truncated/OriginalSize fields to FetchResult for all fetch methods
- Return truncation info in proxy response (truncated, originalSize, returnedSize)
- Add timeout parameter to fetch_url tool (default 30s, max 120s)
- Increase default maxLength from 5KB to 50KB, allow up to 2MB
- Include _hint in response guiding LLM to retry with larger maxLength

Instead of failing when content exceeds limits, the tool now returns
truncated content with guidance for the LLM to request more if needed.

backend/internal/api/fetcher.go

@@ -29,11 +29,13 @@ const (
 
 // FetchResult contains the result of a URL fetch
 type FetchResult struct {
-	Content     string
-	ContentType string
-	FinalURL    string
-	StatusCode  int
-	Method      FetchMethod
+	Content      string
+	ContentType  string
+	FinalURL     string
+	StatusCode   int
+	Method       FetchMethod
+	Truncated    bool // True if content was truncated due to MaxLength
+	OriginalSize int  // Original size before truncation (0 if not truncated)
 }
 
 // FetchOptions configures the fetch behavior
@@ -401,16 +403,22 @@ func (f *Fetcher) fetchWithChrome(ctx context.Context, url string, opts FetchOpt
 	}
 
 	// Truncate if needed
+	var truncated bool
+	var originalSize int
 	if len(content) > opts.MaxLength {
+		originalSize = len(content)
 		content = content[:opts.MaxLength]
+		truncated = true
 	}
 
 	return &FetchResult{
-		Content:     content,
-		ContentType: "text/html",
-		FinalURL:    finalURL,
-		StatusCode:  200,
-		Method:      FetchMethodChrome,
+		Content:      content,
+		ContentType:  "text/html",
+		FinalURL:     finalURL,
+		StatusCode:   200,
+		Method:       FetchMethodChrome,
+		Truncated:    truncated,
+		OriginalSize: originalSize,
 	}, nil
 }
 
@@ -420,7 +428,6 @@ func (f *Fetcher) fetchWithCurl(ctx context.Context, url string, curlPath string
 		"-sS",                          // Silent but show errors
 		"-L",                           // Follow redirects
 		"--max-time", fmt.Sprintf("%d", int(opts.Timeout.Seconds())),
-		"--max-filesize", fmt.Sprintf("%d", opts.MaxLength),
 		"-A", opts.UserAgent,           // User agent
 		"-w", "\n---CURL_INFO---\n%{content_type}\n%{url_effective}\n%{http_code}", // Output metadata
 		"--compressed",                 // Accept compressed responses
@@ -476,16 +483,22 @@ func (f *Fetcher) fetchWithCurl(ctx context.Context, url string, curlPath string
 	fmt.Sscanf(metaLines[2], "%d", &statusCode)
 
 	// Truncate content if needed
+	var truncated bool
+	var originalSize int
 	if len(content) > opts.MaxLength {
+		originalSize = len(content)
 		content = content[:opts.MaxLength]
+		truncated = true
 	}
 
 	return &FetchResult{
-		Content:     content,
-		ContentType: contentType,
-		FinalURL:    finalURL,
-		StatusCode:  statusCode,
-		Method:      FetchMethodCurl,
+		Content:      content,
+		ContentType:  contentType,
+		FinalURL:     finalURL,
+		StatusCode:   statusCode,
+		Method:       FetchMethodCurl,
+		Truncated:    truncated,
+		OriginalSize: originalSize,
 	}, nil
 }
 
@@ -541,17 +554,23 @@ func (f *Fetcher) fetchWithWget(ctx context.Context, url string, wgetPath string
 	content := stdout.String()
 
 	// Truncate content if needed
+	var truncated bool
+	var originalSize int
 	if len(content) > opts.MaxLength {
+		originalSize = len(content)
 		content = content[:opts.MaxLength]
+		truncated = true
 	}
 
 	// wget doesn't easily provide metadata, so we use defaults
 	return &FetchResult{
-		Content:     content,
-		ContentType: "text/html", // Assume HTML (wget doesn't easily give us this)
-		FinalURL:    url,         // wget doesn't easily give us the final URL
-		StatusCode:  200,
-		Method:      FetchMethodWget,
+		Content:      content,
+		ContentType:  "text/html", // Assume HTML (wget doesn't easily give us this)
+		FinalURL:     url,         // wget doesn't easily give us the final URL
+		StatusCode:   200,
+		Method:       FetchMethodWget,
+		Truncated:    truncated,
+		OriginalSize: originalSize,
 	}, nil
 }
 
@@ -599,18 +618,28 @@ func (f *Fetcher) fetchNative(ctx context.Context, url string, opts FetchOptions
 	}
 	defer resp.Body.Close()
 
-	// Read body with limit
-	body, err := io.ReadAll(io.LimitReader(resp.Body, int64(opts.MaxLength)))
+	// Read body with limit + 1 byte to detect truncation
+	body, err := io.ReadAll(io.LimitReader(resp.Body, int64(opts.MaxLength)+1))
 	if err != nil {
 		return nil, fmt.Errorf("failed to read response: %w", err)
 	}
 
+	var truncated bool
+	var originalSize int
+	if len(body) > opts.MaxLength {
+		originalSize = len(body) // Note: this is just maxLength+1, not true original
+		body = body[:opts.MaxLength]
+		truncated = true
+	}
+
 	return &FetchResult{
-		Content:     string(body),
-		ContentType: resp.Header.Get("Content-Type"),
-		FinalURL:    resp.Request.URL.String(),
-		StatusCode:  resp.StatusCode,
-		Method:      FetchMethodNative,
+		Content:      string(body),
+		ContentType:  resp.Header.Get("Content-Type"),
+		FinalURL:     resp.Request.URL.String(),
+		StatusCode:   resp.StatusCode,
+		Method:       FetchMethodNative,
+		Truncated:    truncated,
+		OriginalSize: originalSize,
 	}, nil
 }