docs: add Spring Boot server integration

[soheilqalamkari]

Summary

Adds the community Spring Boot integration to the A2A Java SDK server integrations list.

Project: https://github.com/soheilqalamkari/a2a-spring

Integration capabilities

• Spring Boot 3.5 / Java 17 baseline
• Spring MVC JSON-RPC hosting at /a2a
• Agent Card discovery at /.well-known/agent-card.json
• JSON-RPC task operations and SSE streaming
• Runnable hello-world sample
• Maven build and usage documentation

Verification

• mvn verify passes.
• Official Java client interoperability tests pass.
• The advertised JSON-RPC interface passes the applicable TCK checks: 78/93 JSON-RPC checks passed, with unsupported or optional capabilities skipped.
• TCK instructions and evidence scope: https://github.com/soheilqalamkari/a2a-spring/blob/main/docs/tck.md

Known follow-up items

The Spring integration is not claiming complete upstream conformance yet:

• The AbstractA2AServerTest scaffold is present but remains disabled while the full test-store and queue contract is completed.
• Push notification delivery, persistence/history, authentication, and Maven Central publication remain follow-up work.
• The integration currently advertises JSON-RPC only.

I’m opening this contribution transparently for maintainer feedback on the integration boundary and the remaining conformance work.