Fix TomSelect test helper flake: search queries that can never match

[mroderick]

Summary

Fixes the recurring CI flake where select_from_tom_select fails with expected to find css ".ts-dropdown .option" but there were no matches (e.g. CI on #2851, previously #2740, #2803).

Root cause

Two problems compound:

1. Queries that can never match. Member#full_name includes pronouns ("Jane Doe (she/her)"), and the helper typed it into TomSelect. /admin/members/search only matches CONCAT(name, ' ', surname) and email, so any query containing the pronoun suffix returns no members. Verified by dumping TomSelect state at failure: inputValue: "Ezequiel Bernier (she/her)", dropdown renders "No members found" despite the fetch succeeding.
2. Poisoned query cache. TomSelect caches loads per query (loadedSearches in the vendored tom-select.complete.min.js), so once a query fails or returns empty, that exact query never re-fires. The old helper's 15-second wait could never recover from one bad fetch.

Fix

• Strip the parenthetical from the display name before typing, so queries match what the server searches.
• Wait briefly (5s) for the 3-character results; if none appear, retype the full name as a fresh query — a new cache key forces a new fetch — instead of waiting it out.
• Added a regression scenario in managing_meeting_invitations_spec.rb that aborts the first search request via Playwright route interception, deterministically exercising the retry path.

Testing

• New regression scenario fails on the old helper, passes with the fix.
• parallel_rspec spec/ -n 6: 1342 examples, 0 failures.
• Affected specs (managing_meeting_invitations_spec.rb, meeting_spec.rb) run 5× clean.