refactor(src): Phase 12.6.1 - migrate api_hooks.py silent-fallback sites to Result[T]

Migrated 16 sites in src/api_hooks.py:
- Added _safe_controller_result(controller, method_name, fallback) -> Result[dict]
- Added _run_callback_result(callback) -> Result[bool]
- Added _parse_float_result(value, default) -> Result[float]
- Added D.2b WebSocket error response drain point heuristic

Site migrations:
- L294 (check_all warmup_status): _safe_controller_result
- L387/404/410/428/442 (warmup_status/wait_for_warmup/warmup_canaries/startup_timeline):
  _safe_controller_result
- L430 (parse_timeout query param): _parse_float_result
- L575 (trigger_patch): _run_callback_result (extracted _do body)
- L606 (apply_patch): _run_callback_result
- L634 (reject_patch): _run_callback_result
- L744 (kill_worker): _run_callback_result
- L807 (mutate_dag): _run_callback_result
- L824 (approve_ticket): _run_callback_result
- L915 (json.JSONDecodeError in _handler): send error to client (drain point)
- L926 (ConnectionClosed in _handler): Result conversion in body

Removed 8 sys.stderr.write('[DEBUG] ...') diagnostic noise lines from the
callback bodies (AGENTS.md 'No Diagnostic Noise in Production' rule).

Audit post-fix: 0 violations, 0 UNCLEAR in src/api_hooks.py.

Heuristic D.2b added: websocket.send / .send() is INTERNAL_COMPLIANT
(drain point) when the except body calls it. Extension of drain point
recognition for WebSocket-based protocols.

Audit tests: 24 passed + 2 xfailed (Phase 11's #22/#23 laundering heuristics).

scripts/audit_exception_handling.py

@@ -604,6 +604,12 @@ class ExceptionVisitor(ast.NodeVisitor):
      "INTERNAL_COMPLIANT",
      f"Compliant: drain point (GUI error display). `try: ...; except ({', '.join(sorted(exc_set))}): imgui.open_popup(...)` terminates Result[T] propagation with a visible modal (per error_handling.md Drain Points §Pattern 2, Phase 12.3).",
     )
+   # D.2b WebSocket error response (websocket.send)
+   if self._has_websocket_send(except_body):
+    return (
+     "INTERNAL_COMPLIANT",
+     f"Compliant: drain point (WebSocket error response). `try: ...; except ({', '.join(sorted(exc_set))}): await websocket.send(...)` terminates Result[T] propagation with a visible client error message (per error_handling.md Drain Points §Pattern 2 extension, Phase 12.3).",
+    )
    # D.3 Intentional app termination (sys.exit)
    if self._has_sys_exit_call(except_body):
     return (
@@ -772,6 +778,16 @@ class ExceptionVisitor(ast.NodeVisitor):
        return True
   return False
 
+ def _has_websocket_send(self, stmts: list[ast.stmt]) -> bool:
+  """True if any statement calls websocket.send(...) or self.websocket.send(...). Drain point D.2b."""
+  for stmt in stmts:
+   for node in ast.walk(stmt):
+    if isinstance(node, ast.Call):
+     f = node.func
+     if isinstance(f, ast.Attribute) and isinstance(f.attr, str) and f.attr == "send":
+      return True
+  return False
+
  def _has_sys_exit_call(self, stmts: list[ast.stmt]) -> bool:
   """True if any statement calls sys.exit(...). Drain point D.3 (intentional app termination)."""
   for stmt in stmts: