…ts (NousResearch#34321)

BasePlatformAdapter.extract_media() rejects .md (and 22 other text-based
extensions) that extract_local_files() correctly accepts. This causes
MEDIA:<path> tags pointing to those files to fall through to plain
text on every platform, while a bare path in the same response would
deliver as a native attachment.

Commit ea49b38 narrowed the extract_media regex (to fix Mattermost
false positives) but left out:
  .md .doc .odt .rtf .ods .tsv .json .xml .yaml .yml .ppt .odp
  .key .tar .gz .tgz .bz2 .xz .html .htm .tiff .svg .bmp

Reproduction from issue:
  send_message(message='MEDIA:~/.hermes/cache/documents/report.md',
               target='weixin')
  → before: literal 'MEDIA:~/.hermes/...' delivered as text
  → after:  report.md uploaded as native attachment

Fix:

Extract the extension list into a class-level constant
`BasePlatformAdapter._MEDIA_DELIVERY_EXTS` and use it in BOTH
extract_media() and extract_local_files(). The single source of
truth eliminates the drift class.

Implementation notes:

- _MEDIA_DELIVERY_EXTS is a class attribute (Tuple[str, ...]) so it
  participates in subclass overrides naturally if a platform ever
  needs to narrow it (Mattermost / WhatsApp text-only fallback, etc.)
  without forking extract_media() itself.

- Regex alternation is sorted by length descending so longer
  extensions like `.tgz` win against shorter aliases — necessary
  when the same suffix could otherwise short-circuit a longer one.

- re.IGNORECASE added on extract_media's regex to match the case
  insensitivity already present on extract_local_files. Real
  responses include `MEDIA:/path/foo.PDF` from skill authors who
  uppercase extensions.

- .epub and .opus were already in extract_media but not
  extract_local_files; .apk/.ipa were in extract_media but not
  extract_local_files. Both sets are now in the shared constant,
  which is the union of both prior lists.

Tests (10 new in TestExtractMediaExtensionParity):

- Shared constant exists, is non-empty, contains .md (the issue's
  example).
- Parametrized across all 23 previously-missing extensions: each is
  now in the shared constant AND now matches via extract_media().
- Real-world NousResearch#34321 reproduction case (`MEDIA:~/.hermes/cache/
  documents/report.md`) extracts correctly.
- Cross-method probe: both extract_media and extract_local_files
  accept the same set on real files at /tmp.
- [[audio_as_voice]] still applies to .md (behavior preserved).
- Unknown extensions like .exe still rejected (guardrail: this fix
  expands the allow-list, it does not open the gate to anything).

All 209 tests in test_platform_base.py +
test_extract_local_files.py + test_media_extraction.py pass.

Refs: NousResearch#34321
Closes: NousResearch#34321

Co-authored-by: Cursor <cursoragent@cursor.com>