test: validate PQC TLS cryptography in showcase integration tests

[torreypayne]

Validates Post-Quantum Cryptography (PQC) hybrid key exchange (X25519MLKEM768) across the Showcase integration tests:

• Programmatically generates temporary RSA TLS certificates in shared/test/showcase/test_helper.rb.
• Launches gapic-showcase run with --tls-cert and --tls-key.
• Wires gRPC and REST Showcase clients to connect over TLS with valid channel credentials.
• Adds shared/test/showcase/pqc_test.rb using Fiddle to verify that the underlying TLS socket negotiates quantum-resistant hybrid cryptography.

Stacked on #1330.

Stack

1. chore: update googleads bazel test target to v22 in kokoro #1332: chore: update googleads bazel test target to v22 in kokoro
2. chore: bump gapic-showcase version in test harness to 0.42.0 #1330: chore: bump gapic-showcase version in test harness to 0.42.0
3. test: validate PQC TLS cryptography in showcase integration tests #1331 (This PR): test: validate PQC TLS cryptography in showcase integration tests

Pre-Flight Engineering Audit Sign-Off

• 1. Encapsulation Leaks: No test-only attr_reader/attr_accessor added; internal state inspected via instance_variable_get.
• 2. Concurrency Clarity: Zero raw sleep() calls inside MonitorMixin threads (@pause_cond.wait verified); locks & condition variables explicitly documented.
• 3. Test Brittleness: No private internal deep-stubs; exponential backoff accumulation and gRPC error recovery paths explicitly tested.
• 4. DRY Test Boilerplate: Shared polling loops, mock stubs, and test setup centralized in test/helper.rb; zero unbuffered puts/print noise.
• 5. Native Multi-Version Matrix: Ran bundle exec rake test locally across Ruby 3.2 and Ruby 4.0 (0 failures).