fix(core): clear recently-cancelled request ids on connection close (#892)

Clear the recently-cancelled request id buffer in `doClose` so
connection teardown fully resets per-request bookkeeping.

## Motivation and Context
Follow-up to #884. `doClose` already resets `_responseHandlers`,
`_progressHandlers`, and `inFlightRequestJobs`, but left
`recentlyCancelledRequestIds` populated across a close. This aligns
teardown to clear all Protocol-scoped per-request state.

Not a correctness bug: the buffer is bounded (256) and self-evicting,
request ids are random UUIDs so stale entries cannot false-match a new
connection, and the post-close `onMessage` guard drops late messages
from the old transport before the buffer is consulted. The change
removes the lone asymmetry in `doClose`.

## How Has This Been Tested?
- `./gradlew :kotlin-sdk-core:compileKotlinJvm`
- `./gradlew :kotlin-sdk-core:jvmTest --tests "*ProtocolTest*"`

## Breaking Changes
None.

## Types of changes
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
`recentlyCancelledRequestIds` is Protocol-scoped (like
`_responseHandlers`/`_progressHandlers`), so it is reset in `doClose`
rather than on the per-connection `Connection` object.

kotlin-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/shared/Protocol.kt

@@ -436,6 +436,7 @@ public abstract class Protocol(@PublishedApi internal val options: ProtocolOptio
         }
         val handlersToNotify = _responseHandlers.getAndSet(persistentMapOf()).values.toList()
         _progressHandlers.getAndSet(persistentMapOf())
+        recentlyCancelledRequestIds.getAndSet(persistentListOf())
         connection.inFlightRequestJobs.getAndSet(persistentMapOf())
         connection.handlerScope.cancel()
         onClose()