fix: preserve transcluded media paths in Markdown

[w3lld1]

Summary

• I preserve the source item path when an expanded media transclusion points at a revisioned +get URL.
• I remove the generated do=show query and leading slash before writing editable Markdown.
• I added regression coverage for both audio and video transclusions.

Testing

• pytest -q src/moin/converters/_tests/test_markdown_out.py src/moin/converters/_tests/test_markdown_in_out.py (141 passed)
• pytest -q with the virtual environment on PATH (2031 passed, 17 skipped, 1 xpassed)
• ruff check src/moin/converters/markdown_out.py src/moin/converters/_tests/test_markdown_out.py
• black --check src/moin/converters/markdown_out.py src/moin/converters/_tests/test_markdown_out.py
• git diff --check upstream/master...HEAD

Fixes #1298

[UlrichB22]

The fix still generates a wrong URL in the example mentioned in #2310:

![Your Browser does not support HTML5 audio/video element.](audio.mp3){: page-href="wiki:///help-common/audio.mp3" lang="en" dir="ltr" data-href="/help-common/audio.mp3" class="moin-transclusion"}

The expected markdown is
![audio.mp3](help-common/audio.mp3){: page-href="wiki:///help-common/audio.mp3" lang="en" dir="ltr" data-href="/help-common/audio.mp3" class="moin-transclusion"}

[w3lld1]

Thanks for the concrete example. I pushed 9b500dd5, which now prefers data-href for expanded transclusions even when an xinclude:href is present, and uses the source filename instead of the browser fallback text for expanded audio/video objects.

I added a regression case matching the reported audio shape. Fresh checks on the pushed commit:

• focused Markdown converter suite: 71 passed
• Ruff check on the changed files
• git diff --check