[A2A] D2b: Testcontainers E2E for A2A gateway (issue #5340 follow-up)

[qqeasonchen]

Part of #5296 (Architecture Review, Q4). Follow-up to #5340 / PR #5346 (D2a).

What this issue tracks

D2a (PR #5346) landed the in-process A2A readiness checks:

• TaskExpirer reaper + unit test
• MetaAgentCardRegistry (cluster-shared) + unit test
• A2AGatewayService.createTask try/catch (future-based failure surfacing) + integration test
• docs/a2a-readiness-decision.md (keep A2A Experimental)

D2a pin the contract at the user-facing API. D2b is the Testcontainers-based end-to-end suite that exercises the same scenarios against a live Meta + broker + multi-instance EM deployment. D2b is the gate for promoting A2A from Experimental to Beta (per docs/a2a-readiness-decision.md).

What needs to land

1. **A new module ** under the runtime test sources, gated on a Testcontainers profile (so the default test task stays fast and green).
2. Container composition:
   • Nacos 2.x for the MetaStore (NacosMetaStore is the production wiring)
   • One RocketMQ 5.x broker (or Kafka, depending on what the suite wants to exercise)
   • 3 EventMesh Runtime instances sharing the Nacos Meta
3. Test scenarios (mirror D2a's in-process tests, but end-to-end):
   • Happy path: A2A client on EM-1 submits a task to agent-A registered on EM-2; agent-A completes the task; client receives the response.
   • Failure mode: kill the Nacos container mid-task-submit; the A2A gateway surfaces the failure to the client within a bounded timeout (not hung, not silently dropped).
   • Recovery: restart Nacos; subsequent submits succeed.
   • Multi-instance fence: two EM instances both believe they own agent-A; the Meta CAS + epoch fencing rejects the stale writer.
4. CI: a separate workflow that runs the suite on PRs that touch or . The suite must be green for at least one full release cycle before A2A can be promoted to Beta.

Acceptance

• module exists and is built by Gradle.
• The four scenarios above pass in CI.
• workflow exists and is wired to the correct path filters.
• docs/a2a-protocol/README.md is updated with the post-D2b wire contract and SLOs.
• docs/a2a-readiness-decision.md is updated to record that A2A is promoted from Experimental to Beta.

Depends on / Blocks

• Depends on: [Architecture Review] A2A readiness: TaskExpirer, Meta-backed AgentCard, Runtime E2E, failure testing #5340 D2a (DONE, PR feat(#5340): A2A TaskExpirer + MetaAgentCardRegistry + failure-mode test (D2a) #5346)
• Blocks: closure of [Architecture Review] Consolidate EventMesh boundaries and production-grade consistency #5296 (Q4 must be answered with D2b green)