Backports fixes from #2306 to v1.x to address issue #2328.

When the transport closes while handlers are processing requests (e.g., stdin
EOF during a long-running tool call), the server could crash with
ClosedResourceError when trying to send a response through the already-closed
write stream.

This fix:
1. Wraps the message loop in a try/finally that cancels in-flight handlers
   when the transport closes, preventing them from attempting to respond
2. Catches BrokenResourceError and ClosedResourceError when calling
   message.respond() and logs instead of crashing
3. Properly re-raises transport-close cancellation to let the task group
   handle it (vs client-initiated cancellation which already sent a response)
4. Uses list() snapshot when iterating _response_streams in the finally block
   to avoid 'dictionary changed size during iteration' errors

Fixes #2328

- Hoist SessionMessage/types imports from function bodies to module level
- Add dict[str, Any] type arguments to satisfy pyright strict mode