Concurrent IMAP searches were triggered on every keystroke via
`unawaited(_runSearch(...))`. A slower earlier search completing after
a faster later search would overwrite `_searchResults` with stale data,
causing the wrong email to open when tapping a result.
Fix: add a `_searchGeneration` counter incremented on each search start;
results are only applied when the generation matches (i.e. no newer
search was started since). Clearing the query also resets state
immediately without waiting for any in-flight request.
Added two widget tests:
- tapping the first of multiple search results opens the correct email
- stale results from a slower concurrent search are discarded
Fixes#467
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
