Pre-clean stl_subtitle metadata before sidecar imports; import corrected subs as real language shapes

[KillerX]

Problem

1. Vidispine sidecar subtitle imports fail with "ambiguous path to group: stl_subtitle". Vidispine writes parsed cues into the stl_subtitle metadata group during sidecar import, and the group name resolves to more than one path when the item already carries multiple/stale instances from earlier imports.
2. Corrected subtitles (submitted by the external correction tool via ImportSubtitles) landed only as shape Transcribed_Subtitle_SRT, which exports treat as the AI-generated und fallback gated behind SubsAllowAI — instead of as real Norwegian subtitles.

Changes

Pre-clean before sidecar import

• New vsapi client methods GetMetadataGroupInstances / DeleteMetadataGroupInstances: list every occurrence of a named metadata group across the item's timespans (nested groups included) and remove them via PUT /item/{id}/metadata with <group uuid="..." mode="remove"/> — addressed by uuid, since addressing by name is exactly what Vidispine rejects as ambiguous.
• New DeleteMetadataGroupInstancesActivity (auto-registered via the worker's reflection loop) and constant vscommon.GroupStlSubtitle.
• The pre-clean runs before the sidecar import in ImportSubtitles, ImportSidecarSubtitle (TranscribeVX's detached child), and ImportSubtitlesFromSubtrans — in the last one once before the multi-language loop, so one language's import doesn't wipe the previous one's cues.

Corrected subs as real language shapes

• ImportSubtitles parses input.Language up front (accepts no or nor) and imports the corrected SRT as a third shape sub_<iso>_srt (sub_nor_srt for Norwegian), alongside the existing Transcribed_Subtitle_SRT / transcription_json shapes other consumers rely on. Exports key real subtitles off sub_<iso>_srt, so corrected subs surface as genuine nor subs without SubsAllowAI.

Fail loudly

• ImportSubtitles now waits for the sidecar Vidispine job to complete.
• import_subs.go no longer discards job-wait results with _ =; a failed shape or sidecar job fails the workflow with the language in the error.

Tests

• Unit tests for the removal XML template and the metadata JSON parsing (both MetadataListDocument and bare-document envelopes, nested groups).
• ImportSubtitles suite: asserts all three shape tags and the pre-clean; new sidecar-job-failure path.
• New ImportSubtitlesFromSubtrans suite: cleanup runs exactly once across two languages; a failing sidecar job fails the workflow.

Staging verification

The exact JSON envelope of GET /item/{id}/metadata?group=... should be confirmed against a real item — the parser accepts both shapes, but check the first run's DeletedInstances matches expectations, then confirm a previously failing VXID imports cleanly and a VOD export yields nor subs without SubsAllowAI.

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