fix: lower log level for missing authorization header

[septydev]

✏️ Changes

In DPoP required mode, a missing Authorization header was logged at Error level because it was handled together with invalid header counts.

This change distinguishes the missing-header case from malformed requests:

• requests without an Authorization header are logged at Debug
• invalid header counts continue to be logged at Error
• authentication behavior remains unchanged

A unit test was added for the missing-header case.

🔗 References

Fixes #104

🎯 Testing

• This change adds unit test coverage
• This change adds integration test coverage
• This change has been tested on the latest version of the platform/language

Validated with:

• targeted MessageReceivedHandlerTests: 21 passed
• Release build
• full unit suite: 610 passed, 1 unrelated pre-existing failure
• the same failing cache test reproduces on upstream/master
• git diff --check

✅ Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All changes related to this fix are covered by tests
• All commits are signed