feat(opentelemetry): add push notification tracing and trace-context propagation - #1229

…propagation

- Add OpenTelemetryPushNotificationSenderDecorator with CLIENT spans that
  reflect actual delivery outcome (OK on success, ERROR on failure)
- Add OpenTelemetryA2AHttpClient to inject W3C traceparent/tracestate
  headers into outbound push notification HTTP requests
- Change PushNotificationSender.sendNotification() from void to boolean
  so decorators can observe delivery success or failure
- Use @Alternative @priority(1) to resolve CDI ambiguity on A2AHttpClient
- DRY: merge extractTaskId/extractContextId into single extractIds()
- Fix null safety in span error description
- Remove LENIENT Mockito strictness, stub only in tests that need it
- Refactor integration test helpers: shared HttpClient, Gson, clearExportedSpans()
- Use HashMap instead of ConcurrentHashMap where CopyOnWriteArrayList suffices

Signed-off-by: Emmanuel Hugonnet <ehugonne@redhat.com>