perf(sql-workspace): results wait for a schema lookup on every query; cache table schema #1171

Problem

After every query, GenericSqlWorkspace.execute awaits delegate.resolveTableSchema(userSql, cols) before the rows are shown. For PostgreSQL this calls conn.getTableSchema(...), at least one catalog query, on every SELECT … FROM t, also when the same table was queried a second ago. Over an SSH tunnel or to a remote server this adds a full round trip (or more) to every result.

Scope

• Show rows as soon as executeQuery returns; resolve PKs / types in the background and enable Save / edit hints when they arrive (the tab may already show another result: drop stale answers by generation).
• Cache TableSchema per (connection, database, schema, table) for the session; invalidate on DDL run in the same workspace (DestructiveSqlDetector / classifier already knows DDL) and on Refresh in the tree.
• Same for MySQL and SQLite delegates.

Acceptance

• Rows render without waiting for the schema lookup (test with a fake delegate whose resolveTableSchema never completes).
• Re-running a query on the same table does not query the catalog again; after ALTER TABLE it does.