fix: TomSelect test helper flake — use JS value-set instead of send_keys

[mroderick]

What

Replace send_keys with JS value-set + dispatchEvent('input') in the TomSelect test helper. Bypasses the keystroke synthesis pipeline that can race with TomSelect's 300ms loadThrottle debounce timer under CPU contention in parallel CI.

Why

select_from_tom_select intermittently times out waiting for .ts-dropdown .option in headless CI (observed in PR #2750, Group 1, seed 55042). The existing focus fix (PR #2741) reduced but didn't eliminate the flake.

Root cause: send_keys dispatches 3 rapid keystrokes, each resetting TomSelect's internal setTimeout(300) for the load callback. The debounce timer can starve when parallel test processes contend for CPU, and the AJAX search never fires.

Fix: set input.value and dispatch new Event('input', {bubbles: true}) via execute_script. No focus dependency, no rapid-keystroke race, just one direct mutation that TomSelect's onInput handler processes synchronously.

Verification

• 120/120 passes of all TomSelect feature tests (managing_meeting_invitations_spec.rb + meeting_spec.rb) in 10 sequential runs
• RuboCop clean