Fix charset parsing of quoted HTTP Content-Type parameters

[Andy (jesko2004)]

convert_response() splits Content-Type on every semicolon, including semicolons inside quoted parameter values. For example, text/plain; charset=iso-8859-1; profile="urn:example;charset=utf-8" incorrectly overrides the real charset with UTF-8 and fails to convert b"Caf\xe9".

Use the already imported standard-library email.message.Message to parse the charset parameter. This preserves quoted values and converts the response to Café, without adding a dependency or changing explicit StreamInfo overrides.

Nine regression and control cases use in-memory requests.Response objects and the public conversion API. Against unchanged main, two cases fail and seven pass; all nine pass with this fix.

Related: #1844 handles case-insensitive parameter names. This PR addresses quoted parameter boundaries; the HTTP parser hunk overlaps, while Data URI parsing is unchanged.

Validation (Windows, Python 3.12.14): full core suite 984 passed, 34 skipped with CI skip conditions; related tests 89 passed, 5 skipped; pre-commit run --all-files and git diff --check passed.

Prepared and tested with OpenAI Codex assistance.

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[PRABHU KIRAN VANDRANKI (VANDRANKI)]

Community review, does not clear the merge gate.

I read the diff and confirmed from email.message import Message is already imported at the top of _markitdown.py (line 9, alongside collapse_rfc2231_value from email.utils), so this isn't introducing a new dependency, just reusing an idiom already used elsewhere in the file for header parsing.

I traced the actual bug by hand against the old code:

parts = response.headers["content-type"].split(";")
...
for part in parts:
    if part.strip().startswith("charset="):
        _charset = part.split("=")[1].strip()

For 'text/plain; charset=iso-8859-1; profile="urn:example;charset=utf-8"', the naive .split(";") doesn't know the semicolon inside the quoted profile value isn't a real delimiter, so it produces parts ["text/plain", " charset=iso-8859-1", ' profile="urn:example', 'charset=utf-8"']. The loop first sets charset = "iso-8859-1" from the real parameter, then hits the stray 'charset=utf-8"' fragment (the tail end of the quoted profile string, which happens to start with charset= after stripping) and overwrites it with 'utf-8"', a malformed value with a trailing quote still attached. That's a real, silent charset-detection bug whenever any other Content-Type parameter's quoted value happens to contain the substring charset= after a semicolon.

The fix delegates to email.message.Message.get_param("charset"), which is quote-aware RFC 2045 parameter parsing and doesn't split inside quoted strings. I checked this against all 5 parametrized cases in the new test: charset-before-profile, charset-before-profile-with-extra-params, profile-before-charset, a quoted charset value, and the plain unquoted case. All of them use Caf\xe9 decoded against iso-8859-1 (giving Café), so the test is actually checking the real decoded value, not just that some charset string was extracted, which is a meaningful assertion since decoding with the wrong/malformed charset would produce different or broken output.

I also checked the empty-charset case (charset="") against the isinstance(_charset, str) and _charset.strip() guard: get_param would return "" for an explicitly empty quoted value, .strip() on that is falsy, so charset stays None and falls through to whatever the existing default-charset handling is, same intent as the old if len(_charset) > 0 check.

This is a correct, well-targeted fix using the standard library's own header-parsing logic instead of a hand-rolled split that can't handle quoting.