fix(gateway): support Windows absolute paths in MEDIA tag regex and extract_local_files (#34632)

The MEDIA_TAG_CLEANUP_RE and extract_local_files path regex both used
(?:~/|/) to anchor paths, which only matches Unix-style absolute and
home-relative paths. Two additional _TOOL_MEDIA_RE patterns in run.py
had the same limitation. Windows absolute paths (C:\Users\..., D:/...)
were silently ignored, causing MEDIA directive delivery to fail.

Add [A-Za-z]:[/\\] as a third anchor alternative in all four regex
locations (base.py x2, run.py x2). Also update path separators in
extract_local_files from / to [/\\] so it can traverse Windows
directory trees.

Revert accidental + quantifier in MEDIA_TAG_CLEANUP_RE lookahead
that changed match-one to match-one-or-more (unrelated to fix).

Fixes: #34632

tests/gateway/test_platform_base.py

@@ -361,6 +361,45 @@ class TestExtractMedia:
         assert "[[audio_as_voice]]" not in cleaned
         assert "[[as_document]]" not in cleaned
 
+    # Windows path support — regression coverage for #34632
+
+    def test_media_tag_windows_backslash_path(self):
+        """extract_media should recognise Windows backslash paths."""
+        media, cleaned = BasePlatformAdapter.extract_media(
+            r"MEDIA:C:\Users\kotsu\file.pdf"
+        )
+        assert len(media) == 1
+        assert media[0][0].endswith("file.pdf")
+
+    def test_media_tag_windows_forward_slash_path(self):
+        """extract_media should recognise Windows forward-slash paths."""
+        media, cleaned = BasePlatformAdapter.extract_media(
+            "MEDIA:C:/Users/kotsu/file.pdf"
+        )
+        assert len(media) == 1
+        assert media[0][0].endswith("file.pdf")
+
+    def test_media_tag_windows_drive_root(self):
+        """extract_media should recognise a path at the drive root."""
+        media, cleaned = BasePlatformAdapter.extract_media(
+            r"MEDIA:D:\report.md"
+        )
+        assert len(media) == 1
+        assert media[0][0].endswith("report.md")
+
+    def test_media_tag_unix_paths_still_work(self):
+        """Unix absolute and tilde paths must still extract after Windows change."""
+        for content in ["MEDIA:/tmp/audio.ogg", r"MEDIA:~/docs/notes.md"]:
+            media, _ = BasePlatformAdapter.extract_media(content)
+            assert len(media) == 1, f"Failed for: {content}"
+
+    def test_relative_path_still_ignored(self):
+        """Relative Windows-style paths (no drive letter) must not match."""
+        media, _ = BasePlatformAdapter.extract_media(
+            r"MEDIA:Users\kotsu\file.pdf"
+        )
+        assert media == []
+
 
 class TestMediaExtensionAllowlistParity:
     """Regression coverage for issue #34517 — the MEDIA: extension black hole.