fix: use issuer for JWT client audience

[DaleSeo]

Motivation and Context

The client credentials JWT conformance scenario rejected assertions whose audience defaulted to the token endpoint. Using the authorization server issuer aligns the SDK with the client authentication profile, while retaining the explicit audience override for compatibility.

How Has This Been Tested?

Enabled the auth/client-credentials-jwt conformance suite.

Breaking Changes

None

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed