test: Add bun testing for apiary

[danieljbruce]

Description

This pull request does for apiary what https://github.com/googleapis/google-cloud-node/pull/9208/changes did for google-cloud-node. It adds a CI test that uses bun run test instead of npm test

Adds a new GitHub Actions presubmit workflow (.github/workflows/presubmit-bun.yml) to run the unit test suite against the Bun JavaScript runtime across Node versions 22, 24, and 26.

Key details:

• Sets up Bun 1.3.14 via oven-sh/setup-bun pinned to SHA 0c5077e51419868618aeaa5fe8019c62421857d6 (v2.2.0).
• Uses npm install for dependency management to ensure lockfile and version consistency between CI environments.
• Executes tests with bun run test.

Impact

Ensures ongoing compatibility with the Bun runtime and catches runtime-specific regressions early for users running googleapis in Bun environments. This change is CI-only and introduces no breaking changes or runtime library modifications.

Testing

• Added .github/workflows/presubmit-bun.yml to run on pull requests.
• Validated test matrix against Node 22, 24, and 26 with Bun 1.3.14.

Additional Information

• Dependencies are installed with npm rather than bun install to preserve package resolution consistency with the existing CI pipeline.
• Sets MOCHA_THROW_DEPRECATION: false consistent with existing test jobs.