fix(gateway): hold back half-streamed MEDIA:/[[…]] directives

[santa-sketch]

What does this PR do?

With streaming on (Telegram, transport: edit), a reply that is only a voice note
(MEDIA:/…/cache/audio/tts_….ogg) leaves a visible text bubble with a partial path
(MEDIA:/home/…) above the voice note. Same for [[audio_as_voice]] while it is being typed.

Cause: interim edits fire per chunk, and _clean_for_display only strips complete
deliverable tags (by design, #34517). A tag streamed without its extension yet is shown.
At finish the cleaned final is empty, so _adopt_final_text adopts nothing and the bubble
is never edited again: the partial path stays.

Fix: _should_edit now also defers interim edits while the open (last) line of the cleaned
buffer may still become a directive: an unstripped MEDIA: on that line, a word-start
prefix of MEDIA, or a prefix of [[audio_as_voice]] / [[as_document]]. This mirrors the
existing _is_partial_silence_marker hold-back. The final (non-interim) flush is unchanged,
so undeliverable tags still show up as before.

Related Issue

No existing issue found (searched "MEDIA stream", "streaming media tag", "audio_as_voice").
#73537 fixes the same split-tag class for the API server's SSE endpoints; this is the gateway
stream consumer path.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• gateway/stream_consumer.py: _has_pending_media_directive() + use it in _should_edit().
• tests/gateway/test_stream_consumer_pending_media.py: voice-only reply streamed in chunks,
  a split [[audio_as_voice]], and prose mentioning "MEDIA" (still streams).

How to Test

1. scripts/run_tests.sh tests/gateway/test_stream_consumer_pending_media.py — 2 of the 3 tests fail on main, all pass with the fix.
2. scripts/run_tests.sh over tests/gateway/test_{stream,media,telegram,…voice,…tts,…silence}* (120 files): 1165 passed, 0 failed.
3. Live: Telegram with streaming on, ask for a voice-only reply; no text bubble above the voice note.

Checklist

Code

• I've read the Contributing Guide
• Conventional Commits
• Searched existing PRs
• Only changes related to this fix
• Ran the related gateway test files (above); not the full suite
• python scripts/check passes

🤖 Generated with Claude Code

[100yenadmin]

Seen in production use, and this PR fixes it. Details from the Fish Audio plugin side:

Live report (Hermes v0.21.7, Telegram, streaming: true). The plugin tool fish_speak returns [[audio_as_voice]]\nMEDIA:<path>.ogg. Plugin tools aren't on the gateway's auto-append list, so the model's whole reply was that tag. The voice note arrived, but above it a text message stayed frozen at MEDIA ▉; edit_date was null, so it was never edited. The gateway logged Suppressing normal final send … final delivery already confirmed (streamed=True …).

Why the bubble stays. At finish the cleaned final is empty. _send_or_edit takes the if not visible_stripped: branch, which returns True without editing or deleting the preview unless a native stream is open. So the half-streamed prefix becomes the permanent message. With the 24-character default threshold, [[audio_as_voice]]\nMEDIA is exactly the first send.

Probe. I drove GatewayStreamConsumer with a fake Telegram adapter, default config and chunked deltas, against v0.21.7 source:

Case	v0.21.7	With this PR applied
media-only, interim edits skipped (Telegram's per-chat edit slot busy)	send('\nMEDIA ▉'), never edited	no text message
media-only, interim edits allowed	send('\nMEDIA ▉'), then edits ending on '\nMEDIA:<local path without extension> ▉', which leaves a local path visible	no text message
prose, then the tag	the fragment flashes mid-stream; the final edit clears it	the final edit is 'Here you go.', with no fragment mid-stream

Possible follow-up, not needed for this PR: the empty-final branch could also delete an existing preview. That would cover other cases where a preview was sent and the final cleans to empty.

On the plugin side we're asking the model for one line of text next to the tag (fish-audio-hermes 1.7.4), but this PR is the real fix.