chore: add repro fixture and characterization test for missing-LANGUAGE subtitle bug

[birme]

Summary

• Implements chore: reproduce missing-LANGUAGE subtitle bug with a minimal test vector and config #384 (broken out from BUG: CE Multivariant Manifest Not Returning Correct Subtitle-Track, When Input HLS VOD Is Missing LANGUAGE Attribute #325): commits a minimal repro fixture (NAME-only, no LANGUAGE, subtitles) plus a characterization spec, as a prerequisite for the fix in fix: fall back to NAME attribute for subtitle track matching when LANGUAGE is missing #385.

Fixtures (spec/testvectors/hls_subs_no_language/)

• master.m3u8 — #EXT-X-MEDIA:TYPE=SUBTITLES with NAME="French" and no LANGUAGE.
• video.m3u8, french.m3u8 — minimal video + real-cue subtitle media playlists.
• config.subtitleTracks.json — the exact config.subtitleTracks used (matching-name and mismatched-name variants) with useVTTSubtitles: true.
• evidence_real_cues_name_match.m3u8 / evidence_dummy_segments_name_mismatch.m3u8 — captured subtitle media playlists (real slice segments vs dummy).

Spec (spec/engine/subtitle_missing_language_spec.ts)

Three characterization tests driving the real @eyevinn/hls-vodtolive matching seam (the one session.ts feeds via expectedSubtitleTracks): (1) fixture master truly has NAME but no LANGUAGE; (2) real cues served today when configured name matches the manifest NAME; (3) only dummy segments when name mismatches and there is no LANGUAGE. All assert current behavior, so they pass today; comments flag that #385 will flip case (3).

⚠️ Critical finding (narrows #385 scope)

The bug does not fully reproduce on the pinned @eyevinn/hls-vodtolive@4.1.10 — its matcher already falls back to NAME when LANGUAGE is absent (index.js ~L427-435, case-insensitive against expected language/name). So a NAME-only manifest with a matching config.subtitleTracks[].name already serves real cues. The residual bug only occurs when the manifest has no LANGUAGE and the configured name ≠ manifest NAME. #385 should therefore focus on making channel-engine/matching tolerant of that mismatch (and/or a dependency bump), not on adding a NAME fallback that already exists downstream.

Test plan

• PROOF: npm ci && npm run build && npm test (jasmine) → 131 specs, 0 failures, 7 pending specs

Closes #384

🤖 Automated via Channel Engine Dev daily-backlog-pr skill

[birme]

pr-reviewer verdict: APPROVE (repro/fixture deliverable for #384; the fix is #385)

Reviewed on the merits for a chore/characterization PR — the bar is committed fixtures + an honest, green repro, not the fix itself.

Proofs

• PROOF: npm ci && npm run build → BUILD_OK (tsc --project ./ && cp ./package.json dist/)
• PROOF: npm test → 131 specs, 0 failures, 7 pending specs (EXIT=0)
• PROOF: npx jasmine --filter="Subtitles: source manifest SUBTITLES tag with NAME but no LANGUAGE" → 3 specs, 0 failures, twice (deterministic; jasmine random:true but the specs are self-contained)

Checklist

1. Deterministic passing tests — yes; new spec matches jasmine glob **/*[sS]pec.ts, green on rerun. ✔
2. No trademarks / no commercial names — secret+trademark scan of the diff: NONE. ✔
3. Fixture/spec conventions — fixtures under spec/testvectors/hls_subs_no_language/; spec uses require("@eyevinn/hls-vodtolive") + fs.createReadStream fixtures, mirroring spec/engine/subtitle_spec.ts. ✔
4. Security — read-only test fixtures, no input reaching a live URL/path. ✔
5. Commit — chore: add repro fixture and characterization test for missing-LANGUAGE subtitle bug (closes #384). ✔

Focus items — all confirmed

• Fixture valid. master.m3u8 L9 SUBTITLES #EXT-X-MEDIA has NAME="French" and NO LANGUAGE= — asserted by the spec's own guard it (toContain('NAME="French"') + not.toContain("LANGUAGE=")).
• Real matching seam. Spec drives HLSVod directly with expectedSubtitleTracks / dummySubtitleEndpoint / subtitleSliceEndpoint, mirroring how engine/session.ts builds hlsOpts. Not a mocked fantasy.
• Characterization honest. (a) name-match ({fr, French}) → real cues (/subtitlevtt.vtt, no dummy); (b) name-mismatch + no LANGUAGE ({fr, Francais}) → dummy segments (/dummysubs.vtt, no slice). Both assert TODAY's behavior; the mismatch it is flagged for #385 to flip.
• CRITICAL dependency claim checks out. node_modules/@eyevinn/hls-vodtolive@4.1.10/index.js (~L425-437): if (!item.get("language")) { itemLang = item.get("name"); } — NAME fallback IS already present, and matching accepts element.language OR element.name. So name-match resolves via NAME today; mismatch fails (neither fr nor Francais == French). The note that #385 is narrowed to the name-MISMATCH case is accurate and trustworthy.

Nice touch: committed evidence_*.m3u8 outputs + annotated config.subtitleTracks.json give #385 a ready-made oracle.

No blocking findings. Approving.