feat(server): add onSessionClose hook to ClientConnection

[jskjw157]

Adds an onSessionClose(block: () -> Unit) hook on ClientConnection, delegating to the existing callback chain on ServerSession.onClose.

closes #553

Motivation and Context

Tool, prompt, and resource handlers often acquire session-scoped resources — DB connections, temporary files, background coroutines — that need to be released when the session terminates. Today there's no first-class way for a handler to register that cleanup: the only workaround is to fish the ServerSession out of the Server via sessionId and call its internal onClose, which is awkward and easy to get wrong.

This mirrors the access pattern from #552 (#733): expose what ServerSession already does through the public ClientConnection surface so handlers can reach it directly.

How Has This Been Tested?

Two integration tests in ClientConnectionTest:

• onSessionClose callback runs when the session closes — register from a tool handler, close the client, assert the callback fires.
• multiple onSessionClose callbacks run in registration order — verifies the documented ordering guarantee inherited from ServerSession.onClose.

Only the tool-handler case is exercised because onSessionClose is session-scoped — the registration site (tool vs. prompt vs. resource) doesn't change behavior, and adding three near-identical tests would just be noise.

./gradlew :integration-test:jvmTest --tests "*ClientConnectionTest"
./gradlew :kotlin-sdk-server:apiCheck
./gradlew :kotlin-sdk-server:detekt
./gradlew ktlintCheck

Breaking Changes

None for consumers. Adds one abstract method to ClientConnection, which would require any external implementer to recompile — but ClientConnectionImpl is the only implementation and is internal.

Types of changes

• 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

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[jskjw157]

@devcrocod follow-up to #733 — same access pattern, just delegating to the existing ServerSession.onClose chain. Happy to adjust the API shape (e.g. naming, returning a deregistration handle) if you'd prefer something different.