## Why
Canceled RPC calls could leave entries in the pending request map until further traffic or disconnect. Cleanup should happen when the call is dropped, even if no response arrives.
## What changed
Use a `PendingRequestGuard` to remove each registration when its call completes or is canceled. Protect the pending map with a synchronous mutex so cleanup can run in `Drop`, replacing opportunistic pruning and explicit error-path removals.
## Testing
Add a regression test that drops an in-flight call and verifies that the pending request count immediately reaches zero without further traffic.
GitOrigin-RevId: 82c0629f37a2f4bb89b8d10e167e2a09765b26eb
