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/wecom.py

@@ -910,6 +910,10 @@ class WeComAdapter(BasePlatformAdapter):
         url: str,
         max_bytes: int,
     ) -> Tuple[bytes, Dict[str, str]]:
+        from tools.url_safety import is_safe_url
+        if not is_safe_url(url):
+            raise ValueError(f"Blocked unsafe URL (SSRF protection): {url[:80]}")
+
         if not HTTPX_AVAILABLE:
             raise RuntimeError("httpx is required for WeCom media download")