docs: add Erupt (Spring Boot) to Server Integrations

[erupts]

Draft PR for #891: adds Erupt to the Server Integrations list in the README.

Erupt is an Apache 2.0, annotation-driven Spring Boot 3 framework. This README line describes the target state: AgentCards and AgentExecutors registered from @Erupt / @AiToolbox annotations, plus a built-in admin UI that manages agent cards, MCP servers and per-role tool whitelists.

Current status (why this is a draft):

• The released erupt-ai module (xyz.erupt:erupt-ai, on Maven Central) currently ships the A2A client side only, using the 0.3.x io.a2a SDK via langchain4j-agentic-a2a.
• The server side is being built as a standalone Spring Boot integration module on top of the a2a-java 1.0 server modules (org.a2aproject.sdk).

Checklist from CONTRIBUTING_INTEGRATIONS.md:

• Project page with usage docs and a sample
• Tests extending AbstractA2AServerTest
• TCK passing, with visible results
• Published to Maven Central (xyz.erupt); the new server module will be published there too

I'll fill in the links and mark this PR ready for review once those land. Feedback on the approach is welcome before then, especially on whether AbstractA2AServerTest needs adjusting for a Spring Boot runtime.

Refs #891

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