test: lock-protect fakeSender so -race CI passes

The CI runs go test with -race; the agent runner has two pump goroutines
(pumpStdout + pumpStderr) writing through the sender concurrently, and
the unprotected fakeSender slice append raced. The cancel_test had a
local 'safeSender' workaround for the same issue; promote that mutex
onto fakeSender itself so every test in the package is race-clean
without per-test variants.

- fakeSender grows mu sync.Mutex; Send takes/releases. New snapshot()
  helper for tests that want a stable copy.
- cancel_test drops its local safeSender + sync import; uses fakeSender.

Verified: go test -race ./... passes across all packages.

internal/agent/runner/cancel_test.go

@@ -3,35 +3,16 @@ package runner
 import (
 	"context"
 	"strings"
-	"sync"
 	"testing"
 	"time"
 
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/api"
 )
 
-// safeSender is a thread-safe variant of fakeSender. The cancel test
-// has the runner goroutine sending envelopes while the test goroutine
-// is reading the slice, so we need a mutex.
-type safeSender struct {
-	mu   sync.Mutex
-	envs []api.Envelope
-}
-
-func (s *safeSender) Send(e api.Envelope) error {
-	s.mu.Lock()
-	s.envs = append(s.envs, e)
-	s.mu.Unlock()
-	return nil
-}
-
-func (s *safeSender) snapshot() []api.Envelope {
-	s.mu.Lock()
-	defer s.mu.Unlock()
-	out := make([]api.Envelope, len(s.envs))
-	copy(out, s.envs)
-	return out
-}
+// (fakeSender is defined in runner_test.go; it's already lock-protected
+// because the runner's stdout + stderr pump goroutines call Send
+// concurrently. The original local 'safeSender' here was a workaround
+// from before fakeSender itself grew the mutex.)
 
 // TestRunBackupCanceledMidRunReportsCanceled spawns a backup against
 // a fake restic that sleeps for 30 seconds, cancels the context after
@@ -48,7 +29,7 @@ func TestRunBackupCanceledMidRunReportsCanceled(t *testing.T) {
 	// SIGKILL fallback path firing — slower and noisier.
 	bin := setupScript(t, `exec sleep 30`)
 
-	tx := &safeSender{}
+	tx := &fakeSender{}
 	r := New(Config{ResticBin: bin}, tx, 0)
 
 	ctx, cancel := context.WithCancel(context.Background())