refactor: sidebar architecture - single source of truth, explicit loading states

[datlechin]

Summary

Complete architectural refactor of the sidebar table loading, database switching, and schema provider caching.

Before: 3 sources of truth for tables (session.tables, schema provider cache, LiveTableFetcher), race conditions during database switch, notification-based coupling, workaround flags (isSwitchingDatabase, Task.yield(), isRestoringSelection).

After: Single source of truth (session.tables), explicit SidebarLoadingState enum, coordinator-driven table fetching, no races.

What changed

• SidebarLoadingState enum (.idle, .loading, .loaded, .error) replaces 3 implicit booleans (isLoading, isSwitchingDatabase, hasActiveConnection)
• refreshTables() on coordinator replaces fire-and-forget reloadSidebar() Task
• resetForDatabase() on SQLSchemaProvider replaces triple-call loadSchema() + invalidateTables() + invalidateCache()
• Database switch is now a single linear pipeline: clear UI → driver switch → fetch → atomic update. No Task.yield(), no defer trap
• SidebarViewModel stripped to pure presentation layer (batch ops, search, clipboard). Table fetching removed.
• Notification decoupling: handleDatabaseDidConnect() no longer triggers reloadSidebar()

Removed (dead code / anti-patterns)

• LiveTableFetcher struct and TableFetcher protocol
• isRestoringSelection re-entrancy flag
• onChange(of: tables) auto-reload from stale cache
• invalidateCache(), invalidateTables(), fetchFreshTables() on SQLSchemaProvider
• LiveTableFetcherTests.swift (194 lines)

Stats

• 19 files changed, +132 -781 lines (649 net lines removed)
• 1 test file deleted, 5 test files updated

Test plan

• MySQL: switch databases, verify no table flash, correct tables load
• PostgreSQL: switch databases (reconnects), verify no flash
• Empty database: verify empty state shown (no infinite spinner)
• Reconnect: disconnect/reconnect, verify tables reload
• Autocomplete: verify suggestions work after database switch
• AI chat: verify schema context after switch
• Build succeeds with no warnings