🐛 fix(limiter): distinguish TransactionConflict from ConditionalCheckFailed in _commit_initial (#332)

[sodre]

Summary

• Distinguish TransactionConflict from ConditionalCheckFailed in _is_condition_check_failure() by inspecting CancellationReasons codes instead of treating all TransactionCanceledException the same
• Add _is_transaction_conflict() helper and retry loop with exponential backoff (25ms base, 3 retries) for transient contention errors in _commit_initial()
• Fix _build_retry_failure_statuses() to compute retry_after_seconds from bucket state via calculate_retry_after() instead of hardcoding 0.0
• Apply identical fixes to both async (lease.py) and sync (sync_lease.py) code paths

Test plan

• Unit tests for _is_condition_check_failure(): pure ConditionalCheckFailed, pure TransactionConflict, mixed reasons, no response attribute
• Unit tests for _is_transaction_conflict(): pure conflict, condition-check-only, unrelated exceptions, ClientError fallback
• Unit test: TransactionConflict retries original transaction (not consumption-only path)
• Unit test: TransactionConflict exhausts retries and propagates exception
• Unit test: ConditionalCheckFailed still enters consumption-only retry path (regression)
• Unit test: mixed reasons prefer ConditionalCheckFailed over TransactionConflict
• Unit test: cascade TransactionConflict does not raise false RateLimitExceeded (issue scenario)
• Unit test: _build_retry_failure_statuses() computes non-zero retry_after_seconds from deficit
• Unit test: _build_retry_failure_statuses() returns 0.0 when no deficit
• Run full unit test suite: uv run pytest tests/unit/ -v

Closes #332

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.93%. Comparing base (8e129fe) to head (25a50bb).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #401      +/-   ##
==========================================
+ Coverage   91.89%   91.93%   +0.03%     
==========================================
  Files          33       33              
  Lines        7437     7473      +36     
==========================================
+ Hits         6834     6870      +36     
  Misses        603      603              

Flag	Coverage Δ
doctest	29.41% <27.27%> (-0.03%)	⬇️
e2e	42.31% <38.63%> (-0.06%)	⬇️
integration	51.27% <38.63%> (-0.11%)	⬇️
unit	91.66% <100.00%> (+0.04%)	⬆️

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