stanza: respect context in emittest.Sink (open-telemetry#32733)

**Description:**

Respect context cancellation in emittest.Sink to prevent deadlock if the
emit callback is invoked during manager shutdown.

**Link to tracking Issue:** <Issue number if applicable>

Related to
open-telemetry#32391.
Increasing the poll interval in TestCopyTruncate to 5s reliably causes
the "timeout: expected: 2700, actual: ..." failure in the linked issue;
the test never terminates, and this blocks other tests from running.

**Testing:**

N/A, fixing test code

**Documentation:**

N/A

pkg/stanza/fileconsumer/internal/emittest/sink.go

@@ -57,10 +57,14 @@ func NewSink(opts ...SinkOpt) *Sink {
 	return &Sink{
 		emitChan: emitChan,
 		timeout:  cfg.timeout,
-		Callback: func(_ context.Context, token []byte, attrs map[string]any) error {
+		Callback: func(ctx context.Context, token []byte, attrs map[string]any) error {
 			copied := make([]byte, len(token))
 			copy(copied, token)
-			emitChan <- &Call{copied, attrs}
+			select {
+			case <-ctx.Done():
+				return ctx.Err()
+			case emitChan <- &Call{copied, attrs}:
+			}
 			return nil
 		},
 	}