fix(security): consolidated security hardening — SSRF, timing attack, tar traversal, credential leakage (#5944)

Salvaged from PRs #5800 (memosr), #5806 (memosr), #5915 (Ruzzgar), #5928 (Awsh1).

Changes:
- Use hmac.compare_digest for API key comparison (timing attack prevention)
- Apply provider env var blocklist to Docker containers (credential leakage)
- Replace tar.extractall() with safe extraction in TerminalBench2 (CVE-2007-4559)
- Add SSRF protection via is_safe_url to ALL platform adapters:
  base.py (cache_image_from_url, cache_audio_from_url),
  discord, slack, telegram, matrix, mattermost, feishu, wecom
  (Signal and WhatsApp protected via base.py helpers)
- Update tests: mock is_safe_url in Mattermost download tests
- Add security tests for tar extraction (traversal, symlinks, safe files)

gateway/platforms/api_server.py

@@ -20,6 +20,7 @@ Requires:
 """
 
 import asyncio
+import hmac
 import json
 import logging
 import os
@@ -370,7 +371,7 @@ class APIServerAdapter(BasePlatformAdapter):
         auth_header = request.headers.get("Authorization", "")
         if auth_header.startswith("Bearer "):
             token = auth_header[7:].strip()
-            if token == self._api_key:
+            if hmac.compare_digest(token, self._api_key):
                 return None  # Auth OK
 
         return web.json_response(