[FEATURE REQ] Port hub region caching per partition level from .NET SDK

## Summary

Port the per-partition hub region caching feature from .NET SDK (PR [Azure/azure-cosmos-dotnet-v3#5648](https://github.com/Azure/azure-cosmos-dotnet-v3/pull/5648)) to the Java SDK.

## Background

On single-master accounts encountering repeated 404/1002 (ReadSessionNotAvailable) errors, the SDK should discover the hub region via a 403/3 (WriteForbidden) discovery chain and cache the result per partition. Subsequent requests to the same partition route directly to the cached hub, eliminating redundant discovery round-trips.

## Key Behavior

- After 2 consecutive 404/1002 on a single-master account, set \x-ms-cosmos-hub-region-processing-only\ header
- Non-hub regions return 403/3 (WriteForbidden) — SDK retries to next region (discovery chain)
- Hub region responds with 200 OK — SDK caches hub URI for that partition
- Future requests route directly to cached hub (warm path)
- Works for both PPAF and non-PPAF accounts
- **Must be gated by a feature flag / environment variable** (per Debdatta's guidance)

## .NET Reference

- PR: https://github.com/Azure/azure-cosmos-dotnet-v3/pull/5648
- Key files: ClientRetryPolicy.cs, GlobalPartitionEndpointManagerCore.cs, GatewayStoreModel.cs

## Java SDK Files Likely Affected

- \ClientRetryPolicy.java\
- \GlobalPartitionEndpointManager.java\ / related partition failover classes
- \RxGatewayStoreModel.java\
- \Configs.java\ (feature flag)

## Acceptance Criteria

- [ ] Hub region discovery via 403/3 chain after 2x 404/1002 on single-master
- [ ] Per-partition hub region caching (warm path skips discovery)
- [ ] Feature flag gate (environment variable, disabled by default initially)
- [ ] Works for both PPAF and non-PPAF accounts
- [ ] Unit tests + integration tests