Support non-ASCII passwords in basic_auth

[Bountybette]

Checklist

• I personally discussed the issue with a maintainer before opening this pull request. I understood and followed their requirements.
• I reviewed the design choices and implementation details of the changes. I can explain them without relying on an AI coding agent.
• I wrote a short, specific pull request description that makes it easy for maintainers to understand the problem and the solution.
• I am aware that maintainers have AI coding agents. Merely throwing an AI at an issue wastes their time. It comes across as a lack of respect.

Problem

With basic_auth(credentials=("bob", "pässword")), bob can never log in: the server answers HTTP 500 and logs "process_request failed" with TypeError: comparing strings with non-ASCII characters is not supported, because hmac.compare_digest only accepts ASCII str. The same 500 happens for any client that sends a non-ASCII password for a known username, and for credentials that aren't valid UTF-8, where parse_authorization_basic lets UnicodeDecodeError escape. RFC 7617 test vectors with 123£ already parse fine, so only the comparison breaks.

Solution

Compare UTF-8 encoded bytes in the hard coded check_credentials of the asyncio, sync and legacy implementations (trio reuses the asyncio one), and turn UnicodeDecodeError into InvalidHeaderValue so invalid credentials get a 401. Restricting passwords to ASCII was the alternative, but RFC 7617 specifies UTF-8 and build_authorization_basic already sends UTF-8.

[aaugustin]

Checklist? Second time you ignore it.