test: migrate tests to SQLite database architecture

This commit refactors all tests to work with the new database-based
architecture, replacing the previous JSON config approach.

Key changes:
- Add Database export to lib.rs for test access
- Create test helper functions in support.rs:
  - create_test_state(): Creates empty test state with fresh DB
  - create_test_state_with_config(): Migrates JSON config to DB
- Fix environment isolation in provider_service tests:
  - provider_service_switch_missing_provider_returns_error
  - provider_service_switch_codex_missing_auth_returns_error
- Replace ignored export tests with working alternatives:
  - export_sql_writes_to_target_path (tests Database::export_sql)
  - export_sql_returns_error_for_invalid_path (tests error handling)
- Update error type matching to align with current implementation

All tests now:
- Use isolated test environments (test_mutex + reset_test_fs)
- Access data via Database API instead of RwLock<MultiAppConfig>
- Work with SQLite persistence layer
- Pass without environment pollution or race conditions

Fixes test compilation errors after database migration.

src-tauri/src/lib.rs

@@ -26,6 +26,7 @@ pub use app_config::{AppType, McpApps, McpServer, MultiAppConfig};
 pub use codex_config::{get_codex_auth_path, get_codex_config_path, write_codex_live_atomic};
 pub use commands::*;
 pub use config::{get_claude_mcp_path, get_claude_settings_path, read_json_file};
+pub use database::Database;
 pub use deeplink::{import_provider_from_deeplink, parse_deeplink_url, DeepLinkImportRequest};
 pub use error::AppError;
 pub use mcp::{