fix(gateway): skip MEDIA: tags inside code blocks and blockquotes

extract_media() scanned the full response text without distinguishing
live delivery tags from example paths in fenced code blocks, inline code
spans, and blockquotes. This caused false positives where the agent's
explanation of MEDIA: syntax (or tool output containing example paths)
was stripped from user-visible text and the path was added to the media
delivery list.

Added _mask_protected_spans() helper that replaces protected regions
with equal-length whitespace before regex matching, preserving match
offsets. The helper skips backtick-quoted paths in MEDIA: tags to
maintain existing path extraction behavior.

Fixes #35695

tests/gateway/test_platform_base.py

@@ -400,6 +400,50 @@ class TestExtractMedia:
         )
         assert media == []
 
+    # --- Code block / inline code / blockquote false-positive guards (#35695) ---
+
+    def test_media_in_fenced_code_block_ignored(self):
+        """MEDIA: inside ``` fenced code blocks must not be extracted."""
+        content = "Here is an example:\n```text\nMEDIA:/path/to/example.png\n```\nDone."
+        media, cleaned = BasePlatformAdapter.extract_media(content)
+        assert media == []
+        assert "example" in cleaned.lower()
+
+    def test_media_in_inline_code_ignored(self):
+        """MEDIA: inside backtick inline code must not be extracted."""
+        content = "Use `MEDIA:/path/to/file.png` in your response."
+        media, cleaned = BasePlatformAdapter.extract_media(content)
+        assert media == []
+        assert "MEDIA:" in cleaned  # preserved as text
+
+    def test_media_in_blockquote_ignored(self):
+        """MEDIA: inside a > blockquote must not be extracted."""
+        content = "> To send an image, include MEDIA:/path/to/image.jpg\nEnd."
+        media, cleaned = BasePlatformAdapter.extract_media(content)
+        assert media == []
+        assert "End." in cleaned
+
+    def test_media_outside_code_blocks_still_extracted(self):
+        """Real MEDIA: tags outside protected regions must still work."""
+        content = "MEDIA:/real/file.png\n```code\nMEDIA:/fake/file.png\n```"
+        media, _ = BasePlatformAdapter.extract_media(content)
+        assert len(media) == 1
+        assert media[0][0] == "/real/file.png"
+
+    def test_media_mixed_code_and_prose(self):
+        """Real MEDIA: in prose + example in code block: only prose extracted."""
+        content = (
+            "Here is your file:\n"
+            "MEDIA:/output/report.pdf\n"
+            "Example usage:\n"
+            "```text\nMEDIA:/example/path.pdf\n```\n"
+            "Done."
+        )
+        media, cleaned = BasePlatformAdapter.extract_media(content)
+        assert len(media) == 1
+        assert media[0][0] == "/output/report.pdf"
+        assert "Done." in cleaned
+
 
 class TestMediaInsideSerializedJson:
     """Regression coverage for #34375 — MEDIA: embedded in serialized JSON