test(Gax): add OpenTelemetry tracing unit and Showcase conformance tests

[bshaffer]

Add OpenTelemetry tracing unit tests and Showcase conformance tests covering the Tracing Test Plan (F1–F3):

• Expand , , and with unit tests verifying T4 transport spans ( and ), T3 client spans (), retry resend counts ( and ), error mapping, cancellation, and no-op behavior when no is configured.
• Add running end-to-end Showcase conformance tests over both and transports (including F3.1 and F3.2 retry recovery and retry exhaustion) and wire it into .