fix(ai_client): add 'global' declarations to _set_tool_preset_result

Bug: Phase 11 sites 5+6 migration extracted _set_tool_preset_result and
_set_bias_profile_result helpers. The _set_tool_preset_result helper
modifies _active_tool_preset, _tool_approval_modes, _agent_tools without
declaring them as global, which causes the assignments to create LOCAL
variables instead of modifying the module-level globals.

This regression broke tests/test_bias_integration.py::test_set_tool_preset_with_objects:
    preset = ToolPreset(name='ObjTest', categories={'General': [Tool(name='read_file', approval='auto')]})
    with patch('src.tool_presets.ToolPresetManager.load_all', return_value={'ObjTest': preset}):
        ai_client.set_tool_preset('ObjTest')
    assert ai_client._agent_tools['read_file'] is True
  # Fails: KeyError 'read_file' (the helper created a local _agent_tools,
  # not modifying the module global; set_tool_preset legacy then ran
  # cache-invalidation but never assigned _agent_tools to the test's view)

Fix: Add 'global _active_tool_preset, _tool_approval_modes, _agent_tools'
declaration to _set_tool_preset_result. The original set_tool_preset had
this declaration at the top; the helper extraction lost it.

Audit: no audit change (the helper still classifies as BOUNDARY_CONVERSION
via Heuristic A 'returns Result' pattern).

src/ai_client.py

@@ -524,7 +524,12 @@ def _set_tool_preset_result(preset_name: Optional[str]) -> Result[None]:
  capturing the original exception. The legacy caller (set_tool_preset)
  calls this helper for the load step; on Result errors, the caller still
  completes (state remains partially-set; the cache invalidation runs).
+
+ IMPORTANT: This function MODIFIES module-level globals (_active_tool_preset,
+ _tool_approval_modes, _agent_tools). Without 'global' declarations, the
+ assignments would create local variables that are discarded on return.
  """
+ global _active_tool_preset, _tool_approval_modes, _agent_tools
  if not preset_name or preset_name == "None":
   return Result(data=None)
  try: