Wait for worker query close before session reuse

[bill-ph]

Summary

Fix the worker query close lifecycle so the control plane does not reuse a worker session until the worker has acknowledged that the previous Flight SQL DoGet operation released its server-side operation state.

Previously, FlightRowSet.Close cancelled the worker DoGet stream and returned after releasing the local reader. The worker could still be unwinding its DoGet goroutine and holding the session operation token, so the next same-session query could race into the worker and fail with FailedPrecondition.

This PR adds a small worker WaitSessionIdle action and has FlightRowSet.Close call it after cancelling the stream. That makes close/cancel completion explicit instead of relying on optimistic gRPC cancellation timing.

Changes

• Add WorkerWaitSessionIdlePayload and a WaitSessionIdle worker action.
• Track an operation idle signal on worker sessions and wait on it without polling.
• Make FlightRowSet.Close wait for the worker idle acknowledgement before returning.
• Add deterministic unit coverage for the client close wait and worker action blocking behavior.
• Document why this cleanup-window race is unit-covered rather than asserted in the e2e harness.

Tests

• go test ./server/flightclient -run TestFlightRowSetCloseWaitsForWorkerDoGetCleanup -count=1
• go test ./duckdbservice -run TestWaitSessionIdleBlocksUntilOperationReleases -count=1
• go test ./server/flightclient ./duckdbservice -count=1
• go test ./server/flightsqlingress -count=1
• just lint
• just test-unit