fix: eliminate mutex-copy undefined behaviour in then/eventually.go

[Copilot]

RecordingTestingT embeds sync.Mutex, but was being passed and returned by value in Get(), Merge(), and through the internal channel — all flagged by go vet as mutex-copy violations (undefined behaviour).

Changes

• Latest.latestValue — changed from RecordingTestingT to *RecordingTestingT
• Get() / Merge() — signatures updated to accept/return *RecordingTestingT; Merge gains a nil guard for the first call (before any tick value has been recorded)
• Internal channel — changed from chan RecordingTestingT to chan *RecordingTestingT; goroutine now sends &recordedTesting
• Timer timeout path — nil-checks Get() result before calling FailedTestOutputs(), covering the case where the timer fires before the first assertion completes

// Before
func (l *Latest) Get() RecordingTestingT          // copies mutex
func (l *Latest) Merge(updated RecordingTestingT) RecordingTestingT  // copies mutex twice
channel := make(chan RecordingTestingT, 1)         // send/receive copy mutex

// After
func (l *Latest) Get() *RecordingTestingT
func (l *Latest) Merge(updated *RecordingTestingT) *RecordingTestingT
channel := make(chan *RecordingTestingT, 1)

[coveralls]

coverage: 3.846% (-0.04%) from 3.888%
when pulling 2af6ccb on copilot/security-check-repo-dependencies
into 3601168 on master.