chore: bump gapic-showcase version in test harness to 0.42.0

[torreypayne]

Bumps the GAPIC_SHOWCASE_VERSION constant within shared/test/showcase/test_helper.rb to 0.42.0 to pull in the updated gapic-showcase binary, which natively supports Post-Quantum Cryptography (PQC) and explicit --tls-cert / --tls-key flags.

Stacked on #1332.

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 (This PR): chore: bump gapic-showcase version in test harness to 0.42.0
3. test: validate PQC TLS cryptography in showcase integration tests #1331: 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).