Relax worker session operation admission

[bill-ph]

Summary

This changes the worker session operation model from hard admission control to safety-by-convention at the protocol layer. Pgwire clients do not have a happy-path way to run concurrent user queries on a single connection, but the control plane can still produce brief worker Flight RPC overlap during normal serial flows, especially Describe/GetFlightInfo cleanup followed by Execute. Treating that overlap as an active user operation caused false FailedPrecondition failures.

The worker now keeps the mechanical safety where it matters: connMu serializes actual sql.Conn / sql.Tx access, connWork fences teardown and reapers, and drain tokens still track pending continuations for graceful shutdown. Pending Flight continuations no longer make the session globally busy.

Changes

• Remove the operationOpen session admission flag and make operation helpers only reject closed sessions.
• Change the old busy error path to report session closed instead of session already has an active operation.
• Update worker tests to allow pending GetFlightInfo/DoGet continuations for statements and metadata while preserving drain cleanup assertions.
• Update COPY and reaper tests to rely on connection/drain guards instead of the old admission gate.
• Update the e2e harness README to document why same-session Flight overlap is unit-covered rather than asserted in the in-cluster pgwire harness.

Verification

• Red: go test ./duckdbservice -run "TestStatementFlightInfo.*AllowsNextQueryBeforePriorDoGet" -count=1 failed before the implementation change with session already has an active operation.
• Green: go test ./duckdbservice -count=1
• Green: just test-unit
• Green: git diff --check
• Green: just lint