fix(workspace): make restoreLastSelectedObject connection-type-aware

[ZhuchkaTriplesix]

Motivation & Problem

In #630, restoreLastSelectedObject() was added to MainScreenWorkspaceState to allow returning to the last open table/view when switching back from server overview/stats.

Currently, restoreLastSelectedObject() evaluates cached object references in a static sequential order:

if (lastSelectedPostgresObject != null) { ... }
if (lastSelectedMysqlObject != null) { ... }
if (lastSelectedSqliteObject != null) { ... }
if (lastSelectedExtensionObject != null) { ... }

If lastSelectedPostgresObject was previously set and the active connection is later switched to another engine without clearing or under custom state manipulation, it evaluates Postgres first and could invoke selectPostgresObject() with a non-Postgres connection.

Proposed Scope

1. In MainScreenWorkspaceState.restoreLastSelectedObject():
   • Branch on activeConnection.type.toLowerCase():
     • 'postgres' / 'postgresql': restore lastSelectedPostgresObject
     • 'mysql': restore lastSelectedMysqlObject
     • 'sqlite': restore lastSelectedSqliteObject
     • default / extensions: restore lastSelectedExtensionObject
2. In selectPostgresObject, selectMysqlObject, selectSqliteObject, selectExtensionObject:
   • Clear cached object references belonging to other driver types when setting a new active object.
3. Add unit tests in main_screen_workspace_state_test.dart verifying that restoreLastSelectedObject() restores only objects matching the active connection's driver type.

Acceptance Criteria

• restoreLastSelectedObject() restores only the object matching the active connection's driver type.
• Unit tests verify that restoreLastSelectedObject() ignores cached objects belonging to different drivers.
• All existing workspace state tests continue to pass.