refactor(storage): revamp apiEndpoint handling, enforce uniform authentication, and use PassThroughClient for emulator

[thiyaguk09]

Summary

Refactors apiEndpoint handling in @google-cloud/storage to improve support for Trusted Partner Cloud (TPC) and Private Service Connect (PSC), enforce uniform authentication across all requests, and streamline emulator authentication.

Changes

1. Remove Authentication Bypasses (src/nodejs-common/util.ts, src/nodejs-common/service.ts, src/resumable-upload.ts)

   • Removed the useAuthWithCustomEndpoint and customEndpoint properties and configuration options across MakeAuthenticatedRequestFactoryConfig, ServiceConfig, ServiceOptions, Service, UploadConfig, Upload, and Storage.
   • Removed the conditional bypass in makeAuthenticatedRequestFactory so all requests uniformly authenticate via authClient.authorizeRequest(reqOpts).

2. Revamp apiEndpoint and Add getEndpoint() (src/storage.ts, src/file.ts, src/signer.ts, src/transfer-manager.ts)

   • Changed apiEndpoint visibility on Service and Storage from public to protected.
   • Added async getEndpoint(): Promise<string> on Storage, which resolves the universe domain dynamically via await this.authClient.getUniverseDomain() when no custom apiEndpoint or STORAGE_EMULATOR_HOST was configured at initialization, and returns the sanitized endpoint (https://storage.${universeDomain}).
   • Updated File#generateSignedPostPolicyV4 to await this.storage.getEndpoint().

3. Streamline Emulator Authentication (src/storage.ts)

   • Defaulted options.authClient to a PassThroughClient instance from google-auth-library when process.env.STORAGE_EMULATOR_HOST is set and no custom authClient is provided.

4. Tests (test/index.ts, test/file.ts, test/nodejs-common/util.ts, test/resumable-upload.ts)

   • Removed obsolete unit tests for customEndpoint and useAuthWithCustomEndpoint.
   • Updated Storage unit tests to mock GoogleAuth#getUniverseDomain() when testing await storage.getEndpoint().
   • Added unit test verifying PassThroughClient is used when STORAGE_EMULATOR_HOST is set without an explicit authClient.

Fixes #7828

[gemini-code-assist]

Code Review

This pull request refactors the storage service's endpoint management by removing explicit customEndpoint and useAuthWithCustomEndpoint flags in favor of a more dynamic getEndpoint() method. This change allows for better handling of universe domains and custom endpoints. My review identified several areas for improvement: adding defensive checks when calling getUniverseDomain to prevent runtime errors, ensuring that getEndpoint() is awaited in the URLSigner to maintain correct domain resolution in TPC/PSC environments, and improving test robustness by ensuring stubs are restored in finally blocks to avoid test pollution.