test(payments): re-enable bearer token integ tests (AgentCredentialProviderService fixed)

[aidandaly24]

Summary

Reverts #499, which added @pytest.mark.skip to three bearer-token (CUSTOM_JWT) integration tests while the AgentCredentialProviderService was returning 500s:

• TestPaymentManagerBearerTokenAuth::test_bearer_token_real_api_call
• TestPaymentManagerBearerTokenAuth::test_token_provider_real_api_call
• TestPaymentManagerBearerTokenAuth::test_bearer_create_session_real_api_call

The underlying service issue is now resolved, so the skips are no longer needed.

Why it's safe to re-enable

The original failure was a service-side defect, not a test or SDK bug: CreatePaymentSession / ListPaymentInstruments with a valid Cognito bearer token against a READY CUSTOM_JWT Payment Manager returned InternalServerException: AgentCredentialProviderServiceException (the post-validation JWT→workload-identity exchange was 5xx-ing). It was reproducible across two independent accounts in us-west-2 and began ~May 29.

Verification that it's fixed:

• Live repro now succeeds — a fresh CUSTOM_JWT manager + Cognito client_credentials token returns a valid paymentSessionId, with the service correctly deriving userId from the JWT claim.
• CI is green — the last several Secure Integration test runs on main pass, including the payments job, with no code changes.

Change

Pure revert of #499 — removes the three @pytest.mark.skip decorators. No other changes.