fix(gateway): build _TOOL_MEDIA_RE from shared MEDIA_EXT_ALTERNATION (#37318)

[temalo]

Why

The agent emits MEDIA:/tmp/report.md (or any of a dozen common deliverable extensions). The dispatch-site regex in gateway/platforms/base.py strips the tag and returns {success: true} — but the file is never actually shipped to the user. The reporter found this on Feishu after extensive debugging that ruled out Feishu, streaming, and the platform adapter; the root cause was a regex whitelist mismatch one layer down.

gateway/run.py's auto-append + history-dedup path uses its own _TOOL_MEDIA_RE that hand-rolled the extension list. The list was a strict subset of the canonical MEDIA_DELIVERY_EXTS tuple that gateway/platforms/base.py uses, so the two patterns drifted:

extension	MEDIA_DELIVERY_EXTS	_TOOL_MEDIA_RE (run.py)
.md	✓	✗
.json, .yaml, .yml, .xml	✓	✗
.html, .htm, .svg	✓	✗
.bmp, .tiff	✓	✗
.tsv, .odt, .ods, .odp, .key	✓	✗
.tar, .bz2, .xz	✓	✗

Result: _collect_auto_append_media_tags silently dropped them on the way out, and the in-line copy of the same regex inside the history scan silently dropped them again on the way in.

What

• Export MEDIA_EXT_ALTERNATION (was _MEDIA_EXT_ALTERNATION, kept as a backwards-compat alias) from gateway/platforms/base.py so other dispatch sites can build their regex off the same source-of-truth alternation that drives MEDIA_TAG_CLEANUP_RE and extract_media.
• Rebuild gateway/run.py's module-level _TOOL_MEDIA_RE from that shared alternation.
• Replace the duplicate, locally-redeclared _TOOL_MEDIA_RE inside the history-scan loop with a reference to the module-level pattern, so a future drift can only happen in one place instead of three.
• Add .markdown and .toml to MEDIA_DELIVERY_EXTS (called out by the reporter, lossless additions).
• Explicit comment in base.py noting that .py / .js / .sh are deliberately not in the tuple — the existing test_no_media_extensions contract makes clear that bare-path mentions of arbitrary source files should not auto-ship. The reporter asked for them; the constitution says no, and I preserved the constitution.

Behaviour change footprint

• extract_media (base.py) gains .markdown and .toml as recognised extensions. All other behaviour identical.
• _TOOL_MEDIA_RE in gateway/run.py now matches the full MEDIA_DELIVERY_EXTS set instead of its old subset. This was the bug — fixing the regex strictly broadens what the auto-append path will deliver, which is exactly what the dispatch path already does.
• Two producer-tool layers in gateway/run.py (the _collect_auto_append_media_tags allowlist and the per-tool MEDIA: detection) still gate which tools are eligible to emit MEDIA: tags in the first place, so the broadened regex doesn't open new attack surface — it only stops dropping deliverable tags that the dispatch layer was already approving.

Test coverage

New file: tests/gateway/test_run_tool_media_ext_parity.py

Test class	What it asserts
test_every_delivery_ext_matched_by_run_pattern	Parametric over every extension in MEDIA_DELIVERY_EXTS. Locks in the parity contract so a future contributor who adds an extension to the shared tuple gets a failing test if they forget to keep the patterns in sync.
test_issue_37318_named_extensions_match	Each extension specifically named in the issue (.md, .markdown, .json, .yaml, .yml, .toml, plus the silently-dropped pre-existing ones like .html/.svg/.bmp/...) now matches.
test_run_pattern_uses_shared_alternation	Walks the shared alternation and verifies the run.py regex accepts each branch — defends against re-introducing a hand-rolled subset.
test_existing_paths_still_match	Unix, home-relative, and Windows path forms from the existing #34632 tests still match.
test_invalid_inputs_still_rejected	Relative paths, missing anchor, unknown extension, and bare MEDIA: tokens are still rejected — the fix didn't loosen anchoring or accept arbitrary extensions.
test_md_tag_extracted	End-to-end: BasePlatformAdapter.extract_media strips a MEDIA:/tmp/report.md tag cleanly.

Pre-existing tests (test_run_tool_media_re.py, test_media_extraction.py, test_extract_local_files.py) continue to pass, including test_no_media_extensions which guards the deliberate .py / .log exclusion.

Full tests/gateway/ directory: baseline on upstream/main has 39 failures (pre-existing missing-deps / Python 3.14 / flake — unrelated). My branch has 39 failures, the same set; new failures introduced: 0.

Out of scope

• I did not add .py / .js / .sh to MEDIA_DELIVERY_EXTS even though the issue asked for them, because the existing test_no_media_extensions contract intentionally excludes them from the bare-path auto-detector. Adding them would surprise existing users whose agents mention /tmp/script.py in prose. Users who want source-file delivery can still use the explicit send_message MEDIA: flow with a glob, but the auto-detector should stay narrow. The comment in base.py near MEDIA_DELIVERY_EXTS documents this.
• I did not refactor the broader auto-append flow or the history-dedup path. The fix is the minimum needed: one shared alternation, one module pattern, two call sites using it.
• The issue references "the same issue exists in gateway/run.py (line ~17121)" — that line is the history-dedup _TOOL_MEDIA_RE, and it's the second of the two patterns I fixed.

Fixes #37318

[alt-glitch]

Duplicate of #37342 — both fix #37318 by expanding the MEDIA: delivery extension whitelist (base.py + run.py regexes). Part of the saturated MEDIA-regex cluster; canonical dynamic approach is #29609.

[temalo]

Closing as confirmed duplicate of #37342 per @alt-glitch's triage. Both PRs took the static-whitelist approach to fixing #37318; #29609 has the canonical dynamic fix and is the better path forward.

Thanks for the catch — should've grepped open PRs against the same issue before opening.