feat(session): implement streaming state with text delta and tool‑call diff handling

- Introduce `StreamingMessageState` to track full text, last tool calls, and completion.
- Add `StreamDiff`, `TextDelta`, and `TextDeltaKind` for describing incremental changes.
- SessionController now maintains a `stream_states` map keyed by response IDs.
- `apply_stream_chunk` uses the new state to emit append/replace text deltas and tool‑call updates, handling final chunks and cleanup.
- `Conversation` gains `set_stream_content` to replace streaming content and manage metadata.
- Ensure stream state is cleared on cancel, conversation reset, and controller clear.

crates/owlen-core/src/conversation.rs

@@ -190,6 +190,46 @@ impl ConversationManager {
         Ok(())
     }
 
+    /// Replace the current streaming content for a message.
+    pub fn set_stream_content(
+        &mut self,
+        message_id: Uuid,
+        content: impl Into<String>,
+        is_final: bool,
+    ) -> Result<()> {
+        let index = self
+            .message_index
+            .get(&message_id)
+            .copied()
+            .ok_or_else(|| crate::Error::Unknown(format!("Unknown message id: {message_id}")))?;
+
+        let conversation = self.active_mut();
+        if let Some(message) = conversation.messages.get_mut(index) {
+            message.content = content.into();
+            message.metadata.remove(PLACEHOLDER_FLAG);
+            message.timestamp = std::time::SystemTime::now();
+            let millis = std::time::SystemTime::now()
+                .duration_since(std::time::UNIX_EPOCH)
+                .unwrap_or_default()
+                .as_millis() as u64;
+            message.metadata.insert(
+                LAST_CHUNK_TS.to_string(),
+                Value::Number(Number::from(millis)),
+            );
+
+            if is_final {
+                message
+                    .metadata
+                    .insert(STREAMING_FLAG.to_string(), Value::Bool(false));
+                self.streaming.remove(&message_id);
+            } else if let Some(info) = self.streaming.get_mut(&message_id) {
+                info.last_update = Instant::now();
+            }
+        }
+
+        Ok(())
+    }
+
     /// Set placeholder text for a streaming message
     pub fn set_stream_placeholder(
         &mut self,
@@ -254,7 +294,11 @@ impl ConversationManager {
             .ok_or_else(|| crate::Error::Unknown(format!("Unknown message id: {message_id}")))?;
 
         if let Some(message) = self.active_mut().messages.get_mut(index) {
-            message.tool_calls = Some(tool_calls);
+            if tool_calls.is_empty() {
+                message.tool_calls = None;
+            } else {
+                message.tool_calls = Some(tool_calls);
+            }
         }
 
         Ok(())