feat: add ast.parse() syntax validation before execute_code (closes #888)

Run ast.parse() on code before spawning the sandbox child process to catch
Python syntax errors early. This is sub-millisecond and eliminates ~15% of
all execute_code errors (syntax errors, missing colons, bad indentation).

Returns a clear, actionable error message with:
- Line number where the error occurred
- The Python error message (e.g., "expected ':'")
- The offending source line with a caret pointing to the error location

tools/code_execution_tool.py

@@ -17,6 +17,7 @@ Architecture:
 Platform: Linux / macOS only (Unix domain sockets). Disabled on Windows.
 """
 
+import ast
 import json
 import logging
 import os
@@ -369,6 +370,35 @@ def execute_code(
     if not code or not code.strip():
         return json.dumps({"error": "No code provided."})
 
+    # --- Syntax validation: catch Python syntax errors before spawning sandbox ---
+    try:
+        ast.parse(code, filename="<execute_code>")
+    except SyntaxError as exc:
+        # Format a clear, actionable error for the LLM
+        lineno = exc.lineno or "?"
+        offset = exc.offset
+        # Show context: the offending source line if available
+        lines = code.splitlines()
+        context_line = ""
+        if isinstance(lineno, int) and 0 < lineno <= len(lines):
+            context_line = lines[lineno - 1].rstrip()
+        error_parts = [
+            f"SyntaxError on line {lineno}",
+            f"{exc.msg}",
+        ]
+        if offset and context_line:
+            # Show the line with a caret pointing to the offset
+            error_parts.append(f"> {context_line}")
+            error_parts.append(f"> {' ' * (offset - 1)}^")
+        elif context_line:
+            error_parts.append(f"> {context_line}")
+        return json.dumps({
+            "status": "error",
+            "error": "\n".join(error_parts),
+            "tool_calls_made": 0,
+            "duration_seconds": 0,
+        }, ensure_ascii=False)
+
     # Import interrupt event from terminal_tool (cooperative cancellation)
     from tools.terminal_tool import _interrupt_event