fix(gateway): apply agent.disabled_toolsets in gateway message loop

Widens the cherry-picked fix from @jatingodnani (#17343) to the
gateway path. On main, user_config.agent.disabled_toolsets was only
honored by _get_platform_tools' name-level subtraction — it did not
catch tools pulled in implicitly by a composite toolset (browser
includes web_search, hermes-* platforms include most tools).

Changes:
- gateway/run.py: resolve disabled_toolsets alongside enabled_toolsets
  and pass to AIAgent at both user-facing construction sites (normal
  message loop + single-turn cron-like path). Hygiene/compression
  agents (fixed enabled_toolsets=[memory]) are intentionally untouched.
- gateway/run.py: add (agent, disabled_toolsets) to
  _CACHE_BUSTING_CONFIG_KEYS so editing the list in config.yaml
  invalidates the cached AIAgent on the next message.
- cli.py: drop unused 'import platform' left over from PR #17343's
  import churn; restore 'import sys' used throughout the file.
- model_tools.py: drop unused 'import os, sys' added by PR #17343;
  fix comment reference from #15291 (unrelated OAuth issue) to #17309.

Co-authored-by: jatin godnani <godnanijatin@gmail.com>

cli.py

@@ -15,8 +15,8 @@ Usage:
 
 import logging
 import os
-import platform
 import shutil
+import sys
 import json
 import re
 import concurrent.futures