fix: make safe_url_for_log public, add SSRF redirect guards to base.py cache helpers

Follow-up to Dusk1e's PR #7120 (Slack send_image redirect guard):
- Rename _safe_url_for_log -> safe_url_for_log (drop underscore) since
  it is now imported cross-module by the Slack adapter
- Add _ssrf_redirect_guard httpx event hook to cache_image_from_url()
  and cache_audio_from_url() in base.py — same pattern as vision_tools
  and the Slack adapter fix
- Update url_safety.py docstring to reflect broader coverage
- Add regression tests for image/audio redirect blocking + safe passthrough

tests/gateway/test_platform_base.py

@@ -8,7 +8,7 @@ from gateway.platforms.base import (
     GATEWAY_SECRET_CAPTURE_UNSUPPORTED_MESSAGE,
     MessageEvent,
     MessageType,
-    _safe_url_for_log,
+    safe_url_for_log,
 )
 
 
@@ -25,7 +25,7 @@ class TestSafeUrlForLog:
             "https://user:pass@example.com/private/path/image.png"
             "?X-Amz-Signature=supersecret&token=abc#frag"
         )
-        result = _safe_url_for_log(url)
+        result = safe_url_for_log(url)
         assert result == "https://example.com/.../image.png"
         assert "supersecret" not in result
         assert "token=abc" not in result
@@ -33,15 +33,15 @@ class TestSafeUrlForLog:
 
     def test_truncates_long_values(self):
         long_url = "https://example.com/" + ("a" * 300)
-        result = _safe_url_for_log(long_url, max_len=40)
+        result = safe_url_for_log(long_url, max_len=40)
         assert len(result) == 40
         assert result.endswith("...")
 
     def test_handles_small_and_non_positive_max_len(self):
         url = "https://example.com/very/long/path/file.png?token=secret"
-        assert _safe_url_for_log(url, max_len=3) == "..."
-        assert _safe_url_for_log(url, max_len=2) == ".."
-        assert _safe_url_for_log(url, max_len=0) == ""
+        assert safe_url_for_log(url, max_len=3) == "..."
+        assert safe_url_for_log(url, max_len=2) == ".."
+        assert safe_url_for_log(url, max_len=0) == ""
 
 
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