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/base.py

@@ -124,7 +124,14 @@ async def cache_image_from_url(url: str, ext: str = ".jpg", retries: int = 2) ->
 
     Returns:
         Absolute path to the cached image file as a string.
+
+    Raises:
+        ValueError: If the URL targets a private/internal network (SSRF protection).
     """
+    from tools.url_safety import is_safe_url
+    if not is_safe_url(url):
+        raise ValueError(f"Blocked unsafe URL (SSRF protection): {_safe_url_for_log(url)}")
+
     import asyncio
     import httpx
     import logging as _logging
@@ -232,7 +239,14 @@ async def cache_audio_from_url(url: str, ext: str = ".ogg", retries: int = 2) ->
 
     Returns:
         Absolute path to the cached audio file as a string.
+
+    Raises:
+        ValueError: If the URL targets a private/internal network (SSRF protection).
     """
+    from tools.url_safety import is_safe_url
+    if not is_safe_url(url):
+        raise ValueError(f"Blocked unsafe URL (SSRF protection): {_safe_url_for_log(url)}")
+
     import asyncio
     import httpx
     import logging as _logging