Cosmos: Port region-scoped session token container from Java

### Feature Summary

The region-scoped session token container reduces the scope of sesison tokens to regions applicable for a logical partition key - which reduces the blast radius when there is sustained cross region replication lag for a partition (to only logical partitions that actually used seocndary region)

### Feature Description

[RegionScopedSessionContainer](https://github.com/Azure/azure-sdk-for-java/blob/main/sdk/cosmos/azure-cosmos/src/main/java/com/azure/cosmos/implementation/RegionScopedSessionContainer.java)

Design Doc is here - https://github.com/Azure/azure-sdk-for-java/pull/38003

### Use Case

_No response_

### Alternatives

_No response_

### Additional Context

_No response_

Read the Java RegionScopedSessionContainer implementation linked in the issue and the design document in PR 38003. Then locate the corresponding Cosmos session-token container code in this Rust repository and assess how to port the region-scoped behavior. Done means the container limits session tokens to regions applicable to each logical partition key, reducing the stated replication-lag blast radius.

Written by the indexing model from the issue text.