feat(oauth-client): add RpFactory trait to relying_party.rs
Adds RpFactory trait to support test-driven interactive stage operation.
The trait provides a build() method that returns a RelyingParty instance.
Tests implement RpFactory to construct deterministic RP instances with fixed
seeds for reproducible testing. This trait is used by InteractiveDriveMode
to support both external client flows and in-process RP-driven flows.
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>