Fix the default FrankenPHP Mercure JWT audience

[J3m5]

This fixes the default JWT audience generated for FrankenPHP's built-in Mercure hub.

When neither url nor public_url is configured, the hub is exposed at the root-relative /.well-known/mercure endpoint. The subscriber JWT currently uses that relative URL as its aud claim.

Mercure protocol 1.0 validates the token audience against the hub resource identifier, which is absolute. For an application served from https://app.test, the expected audience is therefore:

https://app.test/.well-known/mercure

This change derives the default audience from the origin of app.url while keeping the hub's public URL itself root-relative.

Explicit claims.aud and public_url values continue to take precedence.

The regression coverage includes:

• application URLs with paths, query strings, and fragments;
• trailing slashes;
• non-default ports;
• IPv6 hosts;
• HTTP localhost URLs;
• explicit public URLs;
• explicit single and multiple audiences;
• a custom issuer.

Validation:

• regression tests fail before the fix as expected;
• 92 tests / 172 assertions pass after the fix;
• targeted Pint and PHPStan checks pass.

This is intended to be merged into the Mercure broadcaster branch used by laravel#61474.