Add JSON-RPC chat adapter (message/send + message/stream)

[lezama]

What

A canonical JSON-RPC chat adapter that makes the existing agents/chat ability drivable by protocol chat clients (e.g. JSON-RPC/SSE chat UIs) — the third integration shape alongside Direct Channels and Remote Bridges.

POST agents-api/v1/agent/{agent_id}, JSON-RPC 2.0:

Method	Transport	Request	Response
message/send	JSON	{jsonrpc, id, method, params}	{jsonrpc, id, result: Task}
message/stream	SSE	same, Accept: text/event-stream	data: {…} frames

How

It's a thin envelope over agents/chat: it reuses the existing handler, conversation store, run-control, and permission stack. The canonical output maps almost 1:1 onto a JSON-RPC Task:

agents/chat output   JSON-RPC Task
------------------   -------------
run_id            -> id
session_id        -> sessionId
reply             -> status.message.parts[0].text
completed===false -> status.state: "input-required" (else "completed")

Token streaming adds one new seam: the wp_agent_chat_stream_handler filter (and a register_chat_stream_handler() helper) — the token-by-token sibling of the synchronous wp_agent_chat_handler. It receives ($input, $emit) and emits canonical deltas (content / tool_call / tool_argument), which the route translates into message/delta SSE frames followed by a terminal result: Task frame.

Graceful degradation: when no streaming runtime is registered, message/stream runs the synchronous handler and emits a single terminal frame — so the endpoint works with any existing handler, just without token-level granularity.

The provider/turn-runner stays out of canonical (consumer territory, the same boundary as agents/chat). End-to-end, the endpoint composes with the opt-in default conversation store (#242) for durable WordPress-native sessions; the store is the persistence example, the endpoint is the transport, and the runtime is supplied by a consumer plugin.

Files

• src/Channels/register-agents-chat-jsonrpc-route.php — the adapter
• tests/agents-chat-jsonrpc-route-smoke.php — 36 assertions (request/response mappers, all three delta wire shapes, frame builders, streaming-handler contract via a mock handler + $emit)
• docs/external-clients.md — new JSON-RPC Protocol Endpoint section
• agents-api.php, composer.json — bootstrap require + smoke registration

Verification

• New smoke: 36/36
• Full smoke suite: green
• PHPStan max: [OK] No errors
• PHPCS via CI (Homeboy lint)

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