fix(GCS+gRPC): correct format Object ids

[coryan]

I was confused about the JSON format for Object ids the wrong synthetic values.

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This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: cc47484bb44d06800420eeaee3671c91f0401fe7

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• Kokoro Logs (requires auth)

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[codecov]

Codecov Report

Merging #7559 (cc47484) into main (5403c48) will increase coverage by 0.03%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main    #7559      +/-   ##
==========================================
+ Coverage   93.28%   93.31%   +0.03%     
==========================================
  Files        1381     1381              
  Lines      119588   119589       +1     
==========================================
+ Hits       111554   111600      +46     
+ Misses       8034     7989      -45     

Impacted Files	Coverage Δ
google/cloud/storage/internal/grpc_client_test.cc	100.00% <ø> (ø)
...oogle/cloud/storage/tests/grpc_integration_test.cc	97.76% <ø> (ø)
...torage/tests/object_basic_crud_integration_test.cc	81.36% <0.00%> (-0.51%)	⬇️
google/cloud/storage/internal/grpc_client.cc	92.04% <100.00%> (+6.59%)	⬆️
google/cloud/pubsub/samples/samples.cc	92.02% <0.00%> (-0.32%)	⬇️
...sub/internal/batching_publisher_connection_test.cc	99.60% <0.00%> (+0.19%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (+0.24%)	⬆️
.../cloud/storage/benchmarks/throughput_experiment.cc	74.87% <0.00%> (+0.50%)	⬆️
.../cloud/storage/testing/storage_integration_test.cc	96.39% <0.00%> (+0.90%)	⬆️
... and 1 more

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