fix(db): a lease released after interrupt decrements the pool entry that replaced it #1213

Part of #1212.

Problem

A lease is released by key, not by the pool entry it was taken from.

• PgLease.release() calls _pool._release(_key) (lib/core/database/postgres_connection_pool.dart:44-49). _release looks up _pool[k] and decrements its refs (:156-169).
• interrupt() removes the entry and force-closes its connection (:173-180 → _removeEntryClosing). Leases taken before still point at the closed connection, but the key is free.
• The next acquire() with that key creates a new entry. When an old lease is released afterwards, _release finds the new entry by key and decrements its refs.

Effects:

• refs of the new entry reaches 0 while a caller still uses it. The idle timer (defaultIdleDisposeDelay, 4 s, :63) then disconnects the session under that caller: Not connected to PostgreSQL on the next statement, seconds after an unrelated action.
• refs can go negative; later releases and the idle timer no longer match the real number of users.

The same code exists in MysqlConnectionPool._release (lib/core/database/mysql_connection_pool.dart:143) and SqliteConnectionPool._release (lib/core/database/sqlite_connection_pool.dart:150).

interrupt() runs often: every table tab dispose (#1215), the SQL editor's Cancel (PostgresSqlExecutionDelegate.cancelQuery, lib/features/postgresql/postgres_sql_workspace.dart:163-171), and the timeout paths.

Scope

• A lease keeps a reference to the _PoolEntry it was given (or the entry's generation number). release() decrements only that entry; if the entry is no longer in the pool, release is a no-op.
• Apply to all three pools. Consider one generic pool implementation for the three drivers, since the bookkeeping is identical.
• Debug assert that refs never goes below 0.

Acceptance

• Unit test per pool (fake connection factory): acquire lease A, interrupt, acquire lease B (new entry), release A → B's refs is still 1 and B is not disconnected after the idle delay.
• Unit test: releasing a lease twice, or after disconnectAll, changes nothing.
• refs cannot become negative (assert covered by a test).