[codex] use server-specific cassette names

[k-kinzal]

Summary

Fixes RECORD/REPLAY cassette collisions by removing the hardcoded cassette file name from server-created interceptors.

Root Cause

Interceptor::initCassette() always used recording, so multiple fake servers sharing a cassette directory wrote to and replayed from the same file.

Changes

• Add cassette name support to Interceptor, ServerOptions, and InterceptorFactory.
• Add Server::withCassetteName() and protected static $CASSETTE_NAME.
• Derive a stable default cassette name from the server class when no explicit name is configured.
• Add OAS_FAKE_CASSETTE_NAME override support.
• Document the new configuration and add regression coverage for default and configured cassette names.

Validation

• composer test
• composer lint