Fix and re-enable RPC tests in CI pipeline

[thomhurst]

Summary

The RunRpcTestsModule in the CI pipeline has been temporarily disabled. The module was previously never running due to mutually exclusive [RunOnLinuxOnly, RunOnWindowsOnly] attributes, so these tests have been silently skipped for some time.

What needs to happen

1. Investigate why RPC tests were originally gated behind OS-specific attributes
2. Fix any underlying test failures
3. Re-enable by restoring TestableFrameworks to return ["net8.0"] (or appropriate frameworks)

Location

• Pipeline module: TUnit.Pipeline/Modules/RunRpcTestsModule.cs
• Test project: TUnit.RpcTests/