test: Enhance o3-pro test to verify model metadata and response parsing

- Add verification that o3-pro model was actually used (not just requested)
- Verify model_used and provider_used metadata fields are populated
- Add graceful handling for error responses in test
- Improve test documentation explaining what's being verified
- Confirm response parsing uses output_text field correctly

This ensures the test properly validates both that:
1. The o3-pro model was selected and used via the /v1/responses endpoint
2. The response metadata correctly identifies the model and provider

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/pii_sanitizer.py

@@ -256,10 +256,27 @@ class PIISanitizer:
         if "content" in sanitized:
             # Handle base64 encoded content specially
             if isinstance(sanitized["content"], dict) and sanitized["content"].get("encoding") == "base64":
-                # Don't decode/re-encode the actual response body
-                # but sanitize any metadata
                 if "data" in sanitized["content"]:
-                    # Keep the data as-is but sanitize other fields
+                    import base64
+
+                    try:
+                        # Decode, sanitize, and re-encode the actual response body
+                        decoded_bytes = base64.b64decode(sanitized["content"]["data"])
+                        # Attempt to decode as UTF-8 for sanitization. If it fails, it's likely binary.
+                        try:
+                            decoded_str = decoded_bytes.decode("utf-8")
+                            sanitized_str = self.sanitize_string(decoded_str)
+                            sanitized["content"]["data"] = base64.b64encode(sanitized_str.encode("utf-8")).decode(
+                                "utf-8"
+                            )
+                        except UnicodeDecodeError:
+                            # Content is not text, leave as is.
+                            pass
+                    except (base64.binascii.Error, TypeError):
+                        # Handle cases where data is not valid base64
+                        pass
+
+                    # Sanitize other metadata fields
                     for key, value in sanitized["content"].items():
                         if key != "data":
                             sanitized["content"][key] = self.sanitize_value(value)