fix: detect truncated tool_calls when finish_reason is not length

When API routers rewrite finish_reason from "length" to "tool_calls",
truncated JSON arguments bypassed the length handler and wasted 3
retry attempts in the generic JSON validation loop. Now detects
truncation patterns in tool call arguments regardless of finish_reason.

Fixes #7680

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

run_agent.py

@@ -9506,12 +9506,41 @@ class AIAgent:
                             invalid_json_args.append((tc.function.name, str(e)))
                     
                     if invalid_json_args:
+                        # Check if the invalid JSON is due to truncation rather
+                        # than a model formatting mistake.  Routers sometimes
+                        # rewrite finish_reason from "length" to "tool_calls",
+                        # hiding the truncation from the length handler above.
+                        # Detect truncation: args that don't end with } or ]
+                        # (after stripping whitespace) are cut off mid-stream.
+                        _truncated = any(
+                            not (tc.function.arguments or "").rstrip().endswith(("}", "]"))
+                            for tc in assistant_message.tool_calls
+                            if tc.function.name in {n for n, _ in invalid_json_args}
+                        )
+                        if _truncated:
+                            self._vprint(
+                                f"{self.log_prefix}⚠️  Truncated tool call arguments detected "
+                                f"(finish_reason={finish_reason!r}) — refusing to execute.",
+                                force=True,
+                            )
+                            self._invalid_json_retries = 0
+                            self._cleanup_task_resources(effective_task_id)
+                            self._persist_session(messages, conversation_history)
+                            return {
+                                "final_response": None,
+                                "messages": messages,
+                                "api_calls": api_call_count,
+                                "completed": False,
+                                "partial": True,
+                                "error": "Response truncated due to output length limit",
+                            }
+
                         # Track retries for invalid JSON arguments
                         self._invalid_json_retries += 1
-                        
+
                         tool_name, error_msg = invalid_json_args[0]
                         self._vprint(f"{self.log_prefix}⚠️  Invalid JSON in tool call arguments for '{tool_name}': {error_msg}")
-                        
+
                         if self._invalid_json_retries < 3:
                             self._vprint(f"{self.log_prefix}🔄 Retrying API call ({self._invalid_json_retries}/3)...")
                             # Don't add anything to messages, just retry the API call