feat(storage): setting idempotency token header

[shubhangi-google]

This pull request introduces support for sending an idempotency token header (X-Goog-Gcs-Idempotency-Token) in API commands, enabled by default via the new add_idempotency_token_header option.
linked storage library Pr : googleapis/google-cloud-ruby#34956

[cpriti-os]

AI review: there are two issues.

1. Global Default Scope Leak (Critical Issue):
   By setting RequestOptions.default.add_idempotency_token_header = true in google/apis/options.rb, this PR enables the GCS-specific X-Goog-Gcs-Idempotency-Token header globally for all 400+ Google API clients using google-apis-core (e.g., YouTube, Drive, Compute Engine, Calendar, Bigtable). This violates service abstraction boundaries and adds a 36-byte UUID overhead and confusing headers to every non-GCS request.
   Fix: Default add_idempotency_token_header to false in google-apis-core and explicitly enable it in the google-apis-storage_v1 client generation or configuration.

2. Redundant Memoization:
   In set_idempotency_token_header, the @idempotency_token ||= SecureRandom.uuid memoization is functionally dead code because of the early return on the line right before it:

return if header.any? { |k, _| k.to_s.downcase == 'x-goog-gcs-idempotency-token' }
@idempotency_token ||= SecureRandom.uuid

Because the header Hash is mutated in place and retained across retries of the ApiCommand instance, the first network attempt injects the header into header. On subsequent retries, the loop evaluates return if header.any? { ... }, successfully finding the original key, and returning early. Therefore, @idempotency_token is never evaluated or used again across retries.
Fix: You can simply do header['X-Goog-Gcs-Idempotency-Token'] = SecureRandom.uuid, as caching the UUID as an instance variable is unnecessary.