fix(db): every PostgreSQL call waits its turn; a wait for the connection has its own limit and message (#1216)

[ZhuchkaTriplesix]

Summary

Finishes #1216 after #1222, which queued execute and kept the session on a 57014 cancel.

• Metadata and stats queries go through the queue. PostgresConnection sent ~30 calls straight to the driver (tables, columns, PKs, stats, version(), the transaction probe…), past our statement queue. The driver starts their default 30 s timeout while they wait for the connection, so one queued behind a long statement could still cancel it. They now run through _queued.
• StatementQueue (core/database/statement_queue.dart) replaces the two copies of the queue in the PostgreSQL and MySQL wrappers. A statement that waits longer than waitLimit (2 min) fails with StatementWaitTimeoutException and never runs; the statement it waited for is not touched.
• Messages. The SQL editor shows Query timed out after 30 s for a statement that ran too long and Waited 120 s for the connection: another statement is still running on it. for one that never got the session. The error mapper (and through it MCP) maps the wait to its own message and hint. A server cancel without a duration keeps its reason (Query timed out: canceling statement due to user request).

Acceptance tests (test/core/database/statement_queue_test.dart):

• after a 57014 cancel, the next statement runs on the same session;
• a statement longer than the next one's timeout is not cancelled, and the next timeout reaches the driver only when that statement starts;
• a bare TimeoutException closes only its own session;
• a MySQL query waiting 11 s (fake time) behind another keeps the session;
• the wait limit fails only the waiting statement, which never runs.

Not in this PR: server-side statement_timeout / max_execution_time for the editor and browse sessions (MCP already has its own since #1217), and SQLite still closes its session on a timeout.

Closes #1216

Test plan

• CI: analysis and all test sections pass (no local runs, per team rule)