fix(security): stop the agent from re-enabling tools an admin disabled (#6357)

* fix(security): stop the agent from re-enabling tools an admin disabled

settings.json:disabled_tools is the only global tool-permission boundary
the Settings > Agent Tools panel offers, yet manage_settings exposed an
`enable_tool` action that removed entries from it. Any agent turn could
undo the admin's restriction, and because the agent reads untrusted web
pages, emails and documents, a planted instruction could re-enable bash,
email or any other tool that had been switched off.

The denylist can now only be narrowed from a turn: `disable_tool` and
`list_tools` keep working, `enable_tool` is refused with a message that
points the user to Settings > Agent Tools, and the action is dropped from
the function schema, the prompt rule and the tool index so models are not
steered into calling it. The free-form set/delete/reset actions were
already unable to reach `disabled_tools`; a test now pins that.

Fixes #5523

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UAB5sz5srMKR7rbeDFtdMH

* fix(settings): close internal API tool-policy bypass

Refuse global tool-policy replacement from internal-token requests even when an admin owner is impersonated, and block the same write in app_api. Route explicit image re-enable requests to list_tools with a Settings pointer. Keep disabled_tools outside general settings actions even if a default is added later. Remove the retired action from capability metadata. Validation: 573 focused tests passed; the running app rejected handler, generic API and encoded-path internal HTTP attempts while preserving human administrator controls.

* fix(mcp): block agent writes to per-server MCP tool policy

PATCH /api/mcp/servers/{id}/tools only checked require_admin, which accepts
the internal tool token, and app_api still forwarded it, so an agent turn
could re-enable MCP tools an admin had switched off. Refuse the internal
token on that route (same check as POST /api/tools) and add PATCH
/api/mcp/servers to the app_api blocklist, with regression tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BCbhLNC7mJsunxqfwwshqn

---------

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
Co-authored-by: Afonso Junior <chatgpt@yack.com.br>

tests/test_tool_policy.py

@@ -3087,7 +3087,7 @@ def test_qwen_explicit_admin_requests_build_safe_args():
     )
     assert al._parse_qwen_explicit_admin_request("turn image generation back on now") == (
         "manage_settings",
-        json.dumps({"action": "enable_tool", "tool": "images"}),
+        json.dumps({"action": "list_tools"}),
     )
     assert al._parse_qwen_explicit_admin_request("list API tokens by name and prefix only") == (
         "manage_tokens",