test: port test_video_handlers.py coverage (handler dispatch, transcripts)

Context

As part of the Slash-n-Burn Built-In XBlocks epic (openedx-platform#37819), the file `lms/djangoapps/courseware/tests/test_video_handlers.py` was deleted from openedx-platform in PR #38750.

This test file covered `VideoBlock` handler logic that now lives in xblocks-contrib. The coverage was not ported before deletion.

What needs to be ported

`test_video_handlers.py` covered:

• Handler dispatch — `handle_ajax`, routing to sub-handlers
• Transcript download/upload/delete — `transcript`, `save_transcript`, `delete_transcript` handlers
• Transcript language/content negotiation — SRT vs SJSON format selection, language fallback
• Video position tracking — `save_user_state` handler
• Metadata save — `metadata_save` handler, field validation

The `video_handlers.py` module in xblocks-contrib is entirely untested at the handler level.

Acceptance Criteria

• Port equivalent tests into `xblocks_contrib/tests/` (or an appropriate subpackage)
• Tests should pass against the xblocks-contrib VideoBlock without importing from xmodule
• CI must remain green

Related

• openedx-platform PR: #38750
• Epic: openedx-platform#37819

Start with video_handlers.py and inspect the deleted lms/djangoapps/courseware/tests/test_video_handlers.py through openedx-platform PR #38750 for the original cases. Port handler-dispatch, transcript, position-tracking, and metadata-save coverage into xblocks_contrib/tests/, then run the test suite and confirm it passes without importing from xmodule.