feat: Durable A2A (Agent2Agent) protocol — client, server, observability

[v1r3n]

Summary

Adds Agent2Agent (A2A) protocol support to Conductor in both directions, with durability as the differentiator: a remote agent call survives a server crash/restart and resumes from persisted execution state.

Two directions

Client — call a remote agent from a workflow (conductor.integrations.ai.enabled=true)

• AGENT (poll / streaming-SSE / push), GET_AGENT_CARD, CANCEL_AGENT.
• Durable: deterministic messageId (stable across retries/restarts), in-flight state lives in the persisted task output (no held thread in poll mode), absolute-deadline + consecutive-poll-failure liveness guards, push backstop poll.

Server — expose any workflow as an A2A agent (conductor.a2a.server.enabled=true)

• One agent per workflow at {basePath}/{workflow}: Agent Card + JSON-RPC message/send / tasks/get / tasks/cancel.
• Idempotent start (messageId → idempotencyKey, RETURN_EXISTING) → server-side effectively-once.
• Multi-turn resume: a follow-up message/send carrying the task id completes the paused HUMAN/WAIT task and resumes the same execution (no duplicate workflow).

Security

SSRF guard on outbound agentUrl (loopback / RFC-1918 / link-local / IPv6 ULA / cloud-metadata — metadata always blocked; redirects disabled); conductor.a2a.client.allow-private-network opt-in for trusted/dev. Optional server api-key (constant-time). Push callbacks use single-use bearer tokens with embedded 24h expiry.

Observability

Micrometer counters via the shared Monitors registry (a2a_client_calls, a2a_client_poll_failures, a2a_rpc_errors, a2a_ssrf_blocked, a2a_server_requests, a2a_server_resumes) + MDC correlation keys across A2A code paths.

Tests

• Unit/wire (MockWebServer), embedded-agent e2e, push callback, server dispatch + multi-turn resume, mapper/worker, observability — 86 ai a2a tests.
• A2ASdkInteropTest — drives the client against the official a2a-sdk reference agent (subprocess): discovery, send, poll, streaming, message-mode. Self-skips when a2a-sdk is unavailable.
• A2ADurableEngineEndToEndTest (test-harness) — AGENT through the real decider + AsyncSystemTaskExecutor + Redis, proving crash/restart resume from persistence. Runs in the existing test-harness CI job (Docker/Redis).

Docs / UI / examples

• docs/devguide/ai/a2a-integration.md (AI Cookbook nav) with worked examples: call/expose, multi-turn resume request/response, push end-to-end.
• ai/examples/*a2a* (call / get-card / server / streaming / push / multi-turn / cancel) + indexed in the examples README; runnable interop and durable demos under ai/src/test/resources/a2a/.
• ui-next: registered AGENT / GET_AGENT_CARD / CANCEL_AGENT (typecheck + build green).
• design/a2a/: protocol study, durability proposal, server design.

⚠️ Follow-up needed (separate, needs workflow token scope)

To make A2ASdkInteropTest run in CI (it self-skips otherwise), add to the build job in .github/workflows/ci.yml after the JDK setup:

      - name: Set up Python (for the A2A interop test)
        uses: actions/setup-python@v5
        with:
          python-version: "3.12"
      - name: Install a2a-sdk (lets A2ASdkInteropTest run against the reference agent)
        run: pip install "a2a-sdk>=0.2,<0.3" uvicorn

(Couldn't be pushed here — the PAT lacks the workflow scope.)

Out of scope (documented follow-ups)

Server-side streaming (message/stream) + push-config endpoints; Agent Card JWS signing/verification (+ signatures/extensions passthrough); getAuthenticatedExtendedCard; per-skill OAuth scopes; gRPC / HTTP+JSON transports; full v1.0 ProtoJSON.

Verify locally

./gradlew :conductor-ai:test --tests '*a2a*'
A2A_PYTHON=<venv>/bin/python ./gradlew :conductor-ai:test --tests '*A2ASdkInteropTest'
./gradlew :conductor-test-harness:test --tests '*A2ADurableEngineEndToEndTest'

Note: inbound A2A server auth (api-key) was moved to the enterprise build; the OSS server is open by default (front with a gateway/firewall). Client→remote auth headers, SSRF guard, push-callback tokens, and cross-agent isolation remain in OSS.