fix(storage): respect MIME message boundary size limits

[coryan]

Fixes #9961

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Base: 94.21% // Head: 94.21% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (f966274) compared to base (6cda215).
Patch coverage: 96.84% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9965   +/-   ##
=======================================
  Coverage   94.21%   94.21%           
=======================================
  Files        1497     1498    +1     
  Lines      140500   140549   +49     
=======================================
+ Hits       132370   132424   +54     
+ Misses       8130     8125    -5     

Impacted Files	Coverage Δ
...loud/storage/internal/generate_message_boundary.cc	81.81% <81.81%> (ø)
...ge/internal/generate_message_boundary_benchmark.cc	97.95% <97.95%> (ø)
google/cloud/storage/internal/curl_client.cc	98.82% <100.00%> (-0.01%)	⬇️
...storage/internal/generate_message_boundary_test.cc	100.00% <100.00%> (ø)
google/cloud/storage/internal/rest_client.cc	73.90% <100.00%> (-0.12%)	⬇️
google/cloud/pubsub/subscriber_connection_test.cc	97.75% <0.00%> (-0.57%)	⬇️
...le/cloud/storage/internal/curl_download_request.cc	89.29% <0.00%> (-0.34%)	⬇️
...cloud/storage/internal/generate_message_boundary.h
...cloud/pubsub/internal/subscription_session_test.cc	98.49% <0.00%> (+0.50%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	83.00% <0.00%> (+2.00%)	⬆️
... and 1 more

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[devbww]

Opinion: I usually think that the minimum is the best value to record (instead of mean or median) for such micro-benchmarks.

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