test: remove --proxyquire-shim from common, bigquery, datastore, and dns

[danieljbruce]

Description

Replaces global proxyquire shim with local test helper in four packages.

Impact

Removes --proxyquire-shim from common, bigquery, datastore, and google-cloud-dns test scripts.

Changes

• Part 2 of 5 in stacked PR series: test: replace proxyquire with sinon in gax, common, error-reporting, translate #9642 -> test: remove --proxyquire-shim from common, bigquery, datastore, and dns #9643 -> test: remove --proxyquire-shim from logging and pubsub packages #9644 -> test: remove --proxyquire-shim from bigtable, spanner, and storage #9645 -> chore: remove proxyquire-bun-shim.cjs and runner flags #9646 (b/570680750).
• Stacked on test: replace proxyquire with sinon in gax, common, error-reporting, translate #9642.
• core/common, handwritten/bigquery, handwritten/datastore, handwritten/google-cloud-dns: Add a runtime-agnostic test/proxyquire.ts helper built on Module.prototype.require and require.cache that works identically in Node.js and Bun.
• Update test imports in those four packages from 'proxyquire' to './proxyquire' (or relative path) and remove --proxyquire-shim from their package.json test scripts.

Testing

• Executed pnpm test under both Node.js and Bun (JS_RUNTIME=bun) in core/common, handwritten/bigquery, handwritten/datastore, and handwritten/google-cloud-dns.
• Verified strict lint and type checks pass via bin/linter.mjs --strict.

Alternatives

• Not merging this pull request was considered, which would keep these four packages coupled to the global bin/proxyquire-bun-shim.cjs preload shim.

[gemini-code-assist]

Code Review

This pull request replaces the external proxyquire dependency with a custom local implementation across multiple packages and updates the test scripts to remove the --proxyquire-shim flag. Feedback on the new proxyquire.ts files highlights a bug in the hasOwn helper, which currently only checks for 'object' types and fails to recognize function-based stubs, thereby ignoring @noCallThru and @global behaviors. It is recommended to update hasOwn to support both 'object' and 'function' types.