fix: Test Connection on saved ds uses Keychain when password is blank

[nkcoder]

Summary

After restarting the app, clicking Test Connection on a saved datasource failed even though the password placeholder claimed it was stored in the macOS Keychain.

LoadConfig deliberately strips passwords from on-disk records, so the edit form opens with an empty password field and the placeholder "stored in macOS Keychain — type to change". The previous handleTest sent that empty string to TestDatasource, which then failed auth — making it look like the Keychain entry was lost. The actual Keychain entry was fine; the Connect path (PingDatasource) loads it correctly.

Fix

• ConnectionManager.tsx: when the form's id matches a saved datasource and the password field is blank, route Test through GoApp.PingDatasource(id) (which reads the password from Keychain on the Go side). New connections, or any case where the user has typed a password, keep the existing TestDatasource path.
• Inner form handleTest now passes buildDs() so the stable id flows through to the outer handler.

Test plan

• npx vitest run — 425 frontend tests pass, including new ConnectionManager test covering the saved-ds-with-blank-password branch
• tsc --noEmit clean
• Manual: wails dev, save a connection, restart, open the connection in edit mode, click Test Connection without typing the password → should succeed
• Manual: same flow, but type a different (wrong) password → should fail with auth error (verifies new password input still goes through TestDatasource)
• Manual: brand-new connection form → Test Connection still works through TestDatasource