Add CHANGELOG entry for 429 throttling early yield fix

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mbhaskar

sdk/cosmos/azure-cosmos/CHANGELOG.md

@@ -8,6 +8,7 @@
 #### Breaking Changes
 
 #### Bugs Fixed
+* Fixed an issue in direct connectivity mode where 429 (Too Many Requests) responses during read/write barrier requests could cause excessive retries instead of yielding early. When all contacted replicas return 429 on consecutive barrier attempts, the SDK now propagates the throttle response (for reads) or returns a 408 with substatus 21013 (for writes) to allow the built-in `ResourceThrottleRetryPolicy` to handle backoff. - See [PR 48914](https://github.com/Azure/azure-sdk-for-java/pull/48914)
 
 #### Other Changes
 * Replaced per-client `Schedulers.newSingle()` schedulers in `GlobalEndpointManager` and `GlobalPartitionEndpointManagerForPerPartitionCircuitBreaker` with shared `BoundedElastic` schedulers in `CosmosSchedulers` to prevent thread count from scaling linearly with client/tenant count. - See [PR 49062](https://github.com/Azure/azure-sdk-for-java/pull/49062)