Fix fragment handling in data URIs

[sean0407]

Summary

• Remove the raw URI fragment before parsing the data payload.
• Preserve percent-encoded # and literal query characters as payload data.
• Add regression coverage for both parse_data_uri and MarkItDown.convert_uri.

Fixes #2690

Verification

• pytest -q packages/markitdown/tests/test_module_misc.py -k data_uri_fragments (12 passed)
• git diff --check

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[PRABHU KIRAN VANDRANKI (VANDRANKI)]

Community review, not a maintainer review and it does not clear the merge gate. I read the full diff and ran it on Windows with Python 3.13.3, against main at 4cc9fa1.

What I ran:

• Called parse_data_uri directly on main and on this branch. On main, data:text/plain,Hello#section returns b'Hello#section', and data:text/plain;base64,SGVsbG8h#AAAA returns b'Hello!\x00\x00\x00', because base64.b64decode silently drops the # and then decodes the AAAA. On the PR the same inputs return b'Hello' and b'Hello!'.
• The new test_data_uri_fragments cases: with the PR's test file on main, 6 of the 12 parametrized cases fail (the three fragment inputs, each through parse_data_uri and through convert_uri). On the PR branch all pass.
• tests/test_module_misc.py and tests/test_cli_misc.py on the PR: 201 passed, 1 skipped, 1 failed. The failure is test_speech_transcription (AudioConverter raises FileNotFoundError here). I did not run it on main, and I did not run the full suite for this PR.

The fix is correct as far as I can tell. A # that is not percent-encoded starts a fragment, and the fragment is not part of the payload (RFC 3986, and the Fetch spec's data: URL processing excludes it too; I am relying on my reading of those, not on a test). The %23 case still decodes to #, which your test covers.

Behavior changes worth knowing about, none of them a blocker in my view:

• Hand-written data URIs with a raw # inside the payload are now truncated at the #. For example data:text/html,<p style="color:#fff">x</p> goes from the full string on main to <p style="color: on the PR. Browsers do the same, but anyone who relied on the old lenient behavior will see silent truncation.
• A # in the header part, such as data:text/plain#frag,Hello or data:#x,Hello, used to parse (with a mimetype of text/plain#frag and #x). It now raises "Malformed data URI, missing ',' separator". That matches the URL-standard behavior, but it is a new error for inputs that used to work.

Only parse_data_uri is changed, and in this repo its only caller is convert_uri (_markitdown.py line 497), so I do not see other code paths affected.