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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