Refactor: Migrate conformance tests to StorageTransport and enhance retry logic

[thiyaguk09]

Description

This PR refactors the GCS conformance test suite to use the low-level StorageTransport.makeRequest architecture instead of high-level SDK abstractions. This change is necessary to ensure that the x-retry-test-id headers are correctly propagated through the transport layer for fault-injection testing.

Key Changes:

• Transport Refactor: Updated conformance-test/libraryMethods.ts (specifically saveResumable, lock, and createBucket) to use the low-level transport.
• Robust Bucket Creation: Implemented "Create-or-Get" logic in createBucket to handle HTTP 409 (Conflict) errors, ensuring tests are resilient to non-empty bucket cleanup failures.
• Dynamic Metageneration: Updated the lock function to dynamically fetch current bucket metadata before applying retention locks, resolving metageneration mismatch errors.
• Header Normalization: Enhanced StorageTransport to consistently lowercase all response header keys, ensuring case-insensitive lookups regardless of the underlying header object type.
• Retry Logic Improvements: Refined the shouldRetry logic to better handle malformed JSON responses and specific GCS error reasons like rateLimitExceeded.

Impact

• Reliability: Fixes several "Scenario 1" and "Scenario 7" failures in the conformance test suite caused by high-level abstraction interference.
• Consistency: Aligns the Node.js library with the expected GCS Resumable Upload protocol and error-handling standards.
• Maintainability: Provides a more predictable path for injecting test-specific headers without modifying the core public API.

Testing

• Unit Tests: Added comprehensive test cases in test/storage-transport.ts to cover shouldRetry branches, including network-level transients (ECONNRESET), malformed JSON strings, and idempotent method logic.
• Coverage: Increased line coverage for storage-transport.ts from ~73% to over 90% by targeting previously uncovered error-handling blocks.
• Integration Tests: Verified against the local conformance test-bench.
• Breaking Changes: None. This refactor only affects internal test utility execution and private transport processing.

Additional Information

The refactor addresses specific issues encountered when running the suite in Node 18+ environments where gaxios and google-auth-library response structures (specifically Headers) behaved differently than in Node 14.

Checklist

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease
• Appropriate docs were updated
• Appropriate comments were added, particularly in complex areas or places that require background
• No new warnings or issues will be generated from this change

Fixes #issue_number_goes_here 🦕