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

[ehsavoie]

• 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

Fixes #1217 🦕