test: port test_video_mongo.py coverage (rendering, student_view, CDN, bumper)

Context

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

This test file covered the VideoBlock rendering layer now residing in xblocks-contrib. The coverage was not ported before deletion.

What needs to be ported

`test_video_mongo.py` covered:

• `get_html` / `student_view` — full render pipeline, context variables, template rendering
• CDN URL rewriting — `rewrite_video_url` logic for video source URLs
• Bumper video — pre-roll video injection, bumper dismissed state
• `editor_saved` — YouTube metadata fetch, sub-import on save
• Public video sharing — `public_access` field rendering
• Transcript availability — `get_transcripts_info`, language selection in context
• Speed/download settings — context variables for player configuration

The xblocks-contrib VideoBlock has no equivalent rendering-layer tests.

Acceptance Criteria

• Port equivalent tests into `xblocks_contrib/tests/`
• 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 by reviewing lms/djangoapps/courseware/tests/test_video_mongo.py and the xblocks-contrib VideoBlock implementation, using the linked deletion PR for the prior coverage. Port the listed rendering, CDN, bumper, editor_saved, sharing, transcript, and player-setting tests into xblocks_contrib/tests/. Done means equivalent tests pass without xmodule imports and CI stays green.