fix: address code review feedback - remove AC references and add JWT assertions
- Issue 1: Remove all AC reference suffixes from assertion message strings in:
- crates/relay/src/routes/oauth_token.rs (11 instances)
- crates/relay/src/db/oauth.rs (5 instances)
- Issue 2: Add iss and aud JWT claim assertions in happy-path tests:
- authorization_code_happy_path_returns_200_with_tokens: assert iss and aud match public_url
- refresh_token_happy_path_returns_200_with_new_tokens: decode access token and assert iss and aud