fix(storage): Set the idempotency token in the implementation of async rewrites

[kalragauri]

In the current implementation, during copy/move operations, if the final step commits on GCS but the client times out, AsyncRetryLoop will automatically retry. GCS will reject the retry as invalid since the session is closed and the rewrite_token is already consumed.

This PR includes an InvocationIdGenerator in RewriterConnectionImpl to generate the token once per Iterate() step, and map it into the ClientContext for all retries of that step.

[gemini-code-assist]

Code Review

This pull request introduces the generation and reuse of an idempotency token during asynchronous rewrite retries in RewriterConnectionImpl. It adds an InvocationIdGenerator member, captures the generated ID in the retry loop lambda, and includes a unit test to verify that the token is reused across retries. The feedback suggests moving the generated ID into the lambda capture using std::move to avoid an unnecessary string copy.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.71%. Comparing base (a899f46) to head (7e7fcd1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16114      +/-   ##
==========================================
- Coverage   92.71%   92.71%   -0.01%     
==========================================
  Files        2353     2353              
  Lines      218986   219029      +43     
==========================================
+ Hits       203031   203066      +35     
- Misses      15955    15963       +8     

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