[codex] Resolve operation-level server URLs

[k-kinzal]

Summary

• Resolve effective OpenAPI server URLs with operation-level servers overriding path-level servers, and path-level servers overriding root servers.
• Register effective server URLs so path/operation-level base URLs can dispatch to the fake server.
• Use most-specific server URL matching for path stripping and prevent an overridden operation from responding through the wrong server URL.
• Generate FakeRequest URLs from the matched operation's effective server URL.

Validation

• composer test
• composer lint