[Bilibili] Fix HTTP 412 extraction and expose DASH audio formats

[zgxkbtl]

Problem

Ordinary Bilibili av/BV extraction relied on the page's embedded
__INITIAL_STATE__. The initial page request can now return HTTP 412, leaving
the state empty and eventually failing with:

TypeError: the JSON object must be str, bytes or bytearray, not NoneType

This can be reproduced with:

you-get -i https://www.bilibili.com/video/BV1nmRcBrEzv/

In addition, DASH audio tracks were only associated with video formats. They
could not be listed or selected directly with -F, and selecting an audio-only
format still triggered irrelevant video URL size probes.

Changes

• Fetch metadata for ordinary av/BV videos from
  /x/web-interface/view, while preserving the existing bangumi and special
  URL flows.
• Validate Bilibili API responses, report useful errors, and select the correct
  cid for multipart videos.
• Use the current shared cookie state and a modern browser user agent for
  Bilibili requests.
• Expose DASH audio qualities 30280, 30232, and 30216 as standalone
  dash-audio-* formats and preserve their .m4a extension.
• Skip video size probes for explicitly selected audio-only formats, and bound
  and close HTTP size-probe requests.
• Add focused unit coverage for API metadata, multipart selection, API errors,
  cookie handling, audio stream selection, and extension handling.

Tests

PYTHONPATH=src uv run --with dukpy python -m unittest discover -s tests -p 'test_bilibili.py'
PYTHONPATH=src uv run --with dukpy python -m unittest discover -s tests -p 'test_common.py'
PYTHONPATH=src uv run --with dukpy python -m unittest discover -s tests -p 'test_extractor.py'
uvx flake8 src tests --count --select=E9,F63,F7,F82 --ignore=F824 --show-source --statistics
uv build

The related unit tests pass (8 tests), the fatal flake8 check reports 0 errors,
and both the sdist and wheel build successfully. Live checks also confirmed that
format listing works for the reproducer and that a selected dash-audio-*
format downloads as an audio-only M4A file.

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