Manual search/ID results are not applied to importer candidates

## Regression

After a successful manual lookup during interactive album import, the importer continues to display the original candidate list rather than the result of the manual lookup.

This appears to be a regression from #6925. The lookup succeeds (for example, logs show `Candidate: ...` followed by `Success. Distance: ...`), but its candidate is not applied to the active import task.

## Impact

For imports that need a manual MusicBrainz search or release ID, this makes beets effectively unusable for tagging: the correct metadata can be fetched but cannot be applied through the candidate prompt.

## Reproduction

1. Start an album import with weak automatic matches.
2. At the candidate prompt, choose **Enter Id** and paste a valid MusicBrainz release ID. (The same occurs with **Enter search**.)
3. Wait for the successful lookup.
4. Observe the candidate prompt.

## Expected behavior

The successful manual search or manual ID lookup replaces the candidates shown for the current task, so the lookup result is available for selection/apply.

## Actual behavior

The prompt still shows the original automatic candidate list, even though the manual lookup completed successfully.

## Scope

This report is intentionally limited to the user-visible regression. It is separate from #6685, which is still unmerged.

Start at the interactive album import candidate prompt and trace the Enter Id and Enter search flows through the manual lookup result. Verify where successful candidates are returned and how the active import task updates its displayed candidates. Done means either manual lookup replaces the original candidate list and the result can be selected and applied, with the regression covered by appropriate tests.

Written by the indexing model from the issue text.