fix(db): closing a table tab force-closes the read-only session other views are using #1215

Part of #1212.

Problem

Closing a table browser tab force-closes the read-only session that other views are using.

• GenericTableView.dispose() calls widget.delegate.cancel(interruptIfBusy: true) (lib/features/workspace/generic_table_view.dart:289).
• Despite the name, the delegates never check whether anything is running. They interrupt the readOnly and the tableWrite slot of the connection and database unconditionally:
  • PostgreSQL: PostgresTableDataDelegate.cancel (lib/features/postgresql/postgres_table_view.dart:238)
  • MySQL: lib/features/mysql/mysql_table_view.dart:237
  • SQLite: lib/features/sqlite/sqlite_table_view.dart:225
• interrupt removes the pool entry and calls forceClose() on its connection. That connection is the same object every readOnly caller of this database holds: the tree catalog, stats dashboards, other open table tabs, the Relations view and MCP calls.

Effects:

• Any of those views that has a statement in flight gets Not connected to PostgreSQL / Not connected to MySQL / a closed-connection error. Closing tab A breaks tab B's page load, the tree expansion, or an MCP run_query running at that moment.
• Every following call reconnects, which runs into the concurrent-connect race (fix(db): concurrent connect() on a pooled connection leaks sockets and SSH tunnels, a failed attempt drops the working session #1214) and the lease release bug (fix(db): a lease released after interrupt decrements the pool entry that replaced it #1213).
• A table Save in flight on tableWrite is cut as well.

Scope

• dispose() releases the lease and never closes a shared session.
• Track in-flight calls per delegate. Only when this delegate has a statement running, cancel that statement:
  • PostgreSQL: a cancel request for the backend (Connection.cancelPendingStatement() in postgres 3.x) — the socket stays open.
  • MySQL: KILL QUERY <connection id> from a separate short-lived connection, or release the lease and let the statement finish.
  • SQLite: release only (statements are local).
• Rename interruptIfBusy or make it do what it says.
• Check the other callers of interrupt / interruptAllModes for the same pattern (workspace close, connection switch) and keep force-close only where the whole connection is meant to go away (disconnect, delete).

Acceptance

• Widget test: two table tabs on the same database; close one while the other loads a page (fake delegate that blocks) → the other one finishes without reconnecting.
• Unit test: dispose of an idle table delegate does not call interrupt / forceClose.
• Unit test: dispose while its own statement runs sends a cancel for that statement and leaves the shared connection connected.