[ENH](log-service): surface LogContentionRetry as backoff

[rescrv]

Description of changes

Map wal3::Error::LogContentionRetry to a ResourceExhausted status
carrying a "contention" backoff-reason so clients back off on
contention the same way they do on batching backpressure.

Test plan

CI + things behave better locally

Migration plan

N/A

Observability plan

N/A

Documentation Changes

N/A

Co-authored-by: AI

[claude]

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[github-actions]

Reviewer Checklist

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Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
• Have all non-obvious changes been commented?

System Compatibility

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Quality

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[blacksmith-sh]

Found 1 test failure on Blacksmith runners:

Failure

Test	View Logs
test_count_to_file_async_attached_function_counts_late_inputs	View Logs

Blacksmith runners detected OOM events on the following jobs:

Job	Details
Rust tests / Integration test ci_k8s_integration 1	View Job
Python tests / test-cluster-rust-frontend (3.9, chromadb/test/property/test_collections_with_database_tenant_ove...	View Job