TranslationManager choose-provider validity fix

TranslationManager: fixed a bug where the create-job "choose provider" step could save a job with no providerKey when a set had no default connector configured and wasn't locked, and only one connector was installed — the step never dispatched a pipeline-valid event, so the Next button stayed enabled while the connector was still resolving asynchronously
Added validity gating based on whether pending.providerKey is actually set, closing the same race for the multi-connector/no-default case too
Merged as PR #174 into v17/main