Summary

Fixes #846.

Some devices, such as Fronius solar inverters, return the non-standard X-WWW-Authenticate header instead of WWW-Authenticate when requesting Digest authentication.

Currently, DigestAuth ignores this header, preventing authentication from proceeding.

This PR adds support for X-WWW-Authenticate as a fallback while preserving existing behaviour:

• Prefer the standard WWW-Authenticate header when both are present.
• Use X-WWW-Authenticate only when the standard header is absent.
• Ignore invalid authentication challenges and non-401 responses.

Testing:

• Added regression tests covering both authentication headers, header precedence and invalid challenges.
• Full test suite passed across Python 3.11 to 3.15.
• Linting, type checking and security analysis passed.
• CodSpeed benchmark tests passed, but uploading results from the fork failed due to a 401 authorisation error.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've reviewed the documentation requirements. No documentation changes are needed because this is a bug fix that introduces no new public API.