What does this PR do?

Makes generated-video delivery model-agnostic, and generalizes the existing
deterministic media auto-append so it covers the whole producer-tool family
instead of just image_generate.

The bug. video_generate returns a structured result —
{"success": true, "video": "<url-or-abs-path>"} (see
agent/video_gen_provider.py success_response) — but it was absent from
both _AUTO_APPEND_MEDIA_TOOL_NAMES and _JSON_MEDIA_TOOL_PATH_FIELDS in
gateway/run.py. So a generated video is delivered only if the model restates
a correctly-formatted MEDIA: tag in its prose reply. When the model omits the
tag, or wraps/mis-formats it (e.g. Markdown emphasis), the file is silently never
attached and the user gets nothing. image_generate already avoids this by
reading its path from the tool's JSON result; videos had no such path.

The fix. Generalize the proven image_generate JSON branch into a small
per-tool field map and add video_generate to the allowlist:

_JSON_MEDIA_TOOL_PATH_FIELDS = {
    "image_generate": ("host_image", "image", "agent_visible_image"),
    "video_generate": ("video",),
}

The path is now read from the tool's structured result, so delivery no longer
depends on how (or whether) the model restates it in free text. This is the
model-agnostic counterpart to the prose-parsing path.

Related Issue

Companion to #45773 (fix(gateway): deliver MEDIA: tags wrapped in Markdown emphasis) — different layer, disjoint files. #45773 hardens the prose-parse
path (extract_media regex in base.py) for hand-authored / pre-existing paths;
this PR fixes the structured-harvest path (auto-append in run.py) for
tool-produced media. They are independent and can land in either order. No
separate issue filed — the gap is fully described above.

Type of Change

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

Changes Made

• gateway/run.py — add "video_generate" to _AUTO_APPEND_MEDIA_TOOL_NAMES;
  turn _JSON_MEDIA_TOOL_PATH_FIELDS into a per-tool field map; generalize the
  tool_name == "image_generate" branch to json_fields = MAP.get(tool_name).
  Docstring/comments updated. No interface, schema, or config change.
• tests/gateway/test_media_extraction.py — video_generate coverage:
  structured path delivered, remote URL ignored, failed generation ignored,
  history dedup, and a guard that the "video" field is only harvested from the
  allowlisted producer tool (a execute_code look-alike result is not
  delivered).

How to Test

1. Before: a video_generate result {"success": true, "video": "/tmp/clip.mp4"}
   with a text-only assistant reply → video not delivered.
2. After: same input → _collect_auto_append_media_tags(...) returns
   ["MEDIA:/tmp/clip.mp4"] → delivered, independent of the model's prose.
3. pytest tests/gateway/test_media_extraction.py -q → all pass (incl. the
   existing image_generate / TTS / stale-leak / compression-fallback cases).

False-positive safety

All four existing guards are reused unchanged:

1. Producer-tool allowlist — only image_generate, video_generate, TTS are
   harvested; generic file producers (write_file, terminal, …) are excluded,
   so an internal/temp file they write is never auto-delivered.
2. Success-gate — payload.get("success") must be truthy.
3. _TOOL_MEDIA_RE — requires a deliverable extension + a real local-path
   anchor, so remote URLs (a provider may return a URL in video) and
   non-media artifacts are rejected (delivery then falls back to the gateway's
   existing URL-download path — unchanged, out of scope here).
4. Current-turn isolation + history dedup — unchanged.

Checklist

Code

• I've read the Contributing Guide
• Commit messages follow Conventional Commits (fix(gateway):)
• I searched for existing PRs (this complements fix(gateway): deliver MEDIA: tags wrapped in Markdown emphasis #45773; not a duplicate)
• My PR contains only changes related to this fix
• I've run pytest tests/ -q — ran the media zone: tests/gateway/test_media_extraction.py, test_platform_base.py, test_run_tool_media_re.py (196 passed, 2 skipped); change is localized to the auto-append path
• I've added tests for my changes
• I've tested on my platform: Debian 13

Documentation & Housekeeping

• Updated docstring/comments in gateway/run.py
• N/A — no config keys changed
• N/A — no architecture/workflow change
• Cross-platform: path anchor + extension guard unchanged; no platform-specific assumptions
• N/A — no tool behavior/schema change