Order inbound requests after prior notifications

[abietic]

Summary

• snapshot the accepted-notification watermark when each inbound request is received
• wait only for notifications at or below that watermark before starting the request handler
• let $/cancel_request interrupt a request waiting at the barrier
• preserve the existing disconnect cleanup contract by starting accepted handlers with a canceled context after the peer is gone

This closes the remaining ordering gap after response-scoped notification barriers: a request could previously overtake notifications that appeared earlier on the same wire. Later notifications do not delay the request, and requests released from the same barrier can still run concurrently.

Verification

• GOTOOLCHAIN=go1.21.13 go test ./...
• GOTOOLCHAIN=go1.21.13 go test -race ./...
• focused inbound/disconnect tests repeated 50 times on the current toolchain
• git diff --check

[abietic]

@ThomasK33 When you have a chance, could you review this follow-up to the response-scoped notification barrier? The root suite and -race both pass with the repository minimum Go 1.21.13 toolchain.