Add comprehensive tests for ForwardParams functionality

Tests verify that parameters from the OAuth start URL are correctly
forwarded to both authorization and token exchange requests. This is
critical for Vanta integration which relies on dynamic source_id
parameter forwarding.

Test coverage:
- Single parameter forwarding (source_id for Vanta)
- Multiple parameter forwarding
- Missing parameters (should not cause errors)
- ForwardParams working alongside static AuthRequestParams/TokenRequestParams

Author: mjbraun