bug: Investigate and re-enable disabled benchmark tests

The following benchmark tests are currently disabled in the macOS Kokoro CI build:

- `//generator/integration_tests:benchmarks_client_benchmark`
- `//google/cloud:options_benchmark`

These tests were disabled in `ci/kokoro/macos/builds/bazel.sh` to resolve CI failures and stabilize the build.

This issue tracks the work to:
1. Investigate the root cause of the failures in these benchmarks.
2. Fix the underlying issues.
3. Re-enable the tests by removing them from the `BAZEL_TEST_EXCLUDES` list in the `ci/kokoro/macos/builds/bazel.sh` script.

Start with the two named benchmark targets and inspect their exclusion in ci/kokoro/macos/builds/bazel.sh, especially the BAZEL_TEST_EXCLUDES list. Reproduce the macOS Kokoro failures and investigate their root causes. Done means the underlying issues are fixed, both benchmarks are removed from the exclusion list, and the macOS CI build passes.

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