feat(server): allow injecting custom A2AHttpClient for push notificat…

[malladinagarjuna2]

Overview

This PR introduces the ability to plug in a custom A2AHttpClient implementation on the server side by natively supporting CDI injection in the BasePushNotificationSender.

Fixes #135

The Problem

When server-related code was extracted from core into server-common (in #129), the Push Notification sender was modified to hardcode its usage of JdkA2AHttpClient via A2AHttpClientFactory.create(). This prevented developers from replacing the underlying HTTP client on the server side (e.g. replacing the JDK client with the Vert.x HTTP client, or providing a custom client with specific interceptors/auth mechanisms).

The Solution

1. Added Instance<A2AHttpClient> to CDI Constructor: The @Inject constructor in BasePushNotificationSender now accepts an Instance<A2AHttpClient>.
2. Graceful Fallback: If the DI framework (like Quarkus or Spring) has a custom A2AHttpClient bean available (!httpClientInstance.isUnsatisfied()), it will use it. Otherwise, it gracefully falls back to A2AHttpClientFactory.create(), ensuring 100% backwards compatibility for users who don't provide a custom bean.