Adding integration oauth tests

[cutecatfann]

What I did

• Add 8 Go integration tests for the CE OAuth flow covering the full happy path (PKCE, state validation, token exchange, token store, revoke), error paths (token exchange failure, callback error/timeout), and single-use state enforcement

Testing

• cd mcp-gateway && go test -count=1 -v ./pkg/oauth/ -run 'TestIntegration_' -- 8/8 pass