test: A2A ITK interoperability harness + baseline (no SDK changes)

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ITK Interoperability Harness + Baseline (showcase — no SDK changes)

This PR adds a test-harness that drives the unmodified A2A Elixir SDK against the official A2A Interoperability Test Kit (ITK), plus an honest capability/gap baseline report. It's a measuring stick, not a fix — zero changes to lib/.

Why

Establish a reproducible baseline so future v1.0-compliance work is gap-driven and regression-gated, and so reviewers can see exactly what the SDK can/can't do against a real A2A client today.

What's added (harness + docs only)

• test/support/itk/instruction.ex — ITK Instruction protobuf codec
• test/support/itk/agent.ex — JSON-RPC handler / instruction interpreter
• test/itk/server.exs — standalone Bandit server (v0.3 card, JSON-RPC, SSE)
• test/itk/{instruction,agent}_test.exs + binary fixtures
• docs/ITK_BASELINE.md — full capability/gap report with reproducible evidence

git diff --stat origin/main -- lib/ is empty (verified).

Baseline summary

✅ Works (404 unit tests green against pristine SDK):

• v0.3-shaped agent card (preferredTransport: JSONRPC, protocolVersion: 0.3.0)
• ITK Instruction proto decode (return_response, steps, call_agent)
• Interpreter + Task construction; non-streaming message/send round-trip

❌ Gaps (vs a2a-sdk 0.3.24 Python client — documented, not fixed here):

• JSON-RPC enum encoding: SDK emits ROLE_AGENT / TASK_STATE_COMPLETED (proto-style); v0.3 client expects agent / completed
• SSE streaming event envelopes: Task snapshot vs the TaskStatusUpdateEvent / TaskArtifactUpdateEvent union (missing taskId/kind)
• Card shape: SDK's native encode_agent_card emits v1.0 supportedInterfaces; v0.3 client wants preferredTransport/additionalInterfaces

Prioritized v1.0 gap list (seeds future, separate PRs)

1. JSON-RPC enum encoding (highest leverage — blocks every traversal)
2. Streaming event envelopes
3. v0.3 agent-card emission
4. gRPC / REST transports (deferred)

Reproduce

mix test                                              # 404 + 2 doctests, 0 failures
MIX_ENV=test mix run test/itk/server.exs --httpPort 10130
# + ITK driver: uv run --no-sources python itk_baseline_elixir.py

Note: the standalone server must run under MIX_ENV=test (harness modules live in test/support/, only on elixirc_paths in test env).

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The plan is: Land this PR with dustin for v0.3 and 1.0. then upgrade.