Rpc Dispatch

Dispatch JSON-RPC 2.0 requests for the unified /rpc POST endpoint.

Supported methods: initialize — MCP-aligned session initialisation (no session header needed) notifications/initialized — client acknowledgement after initialize (requires session header) message/send — send a user message (session header optional for backward compat)

Error envelope (camelCase via AssistantGatewayMixin):

{"jsonrpc": "2.0", "id": <id>, "error": {
    "code": <int>, "message": <str>,
    "data": {"category", "kind", "retryable",
             "resetAt", "retryAfter", "userMessage"}}}

See docs/source/domains/assistant-gateway/concepts/error-contract.md for full field semantics, per-category data.kind registry, and rendering guidance. The rationale lives in Decision 006 (docs/source/domains/assistant-gateway/decisions/decisions-006-error-taxonomy.md).

Error codes — five canonical categories plus the JSON-RPC envelope error:

-32600  Invalid Request — a structurally valid JSON-RPC envelope that is
        not a usable request (unknown method, request missing its id, or
        params/blocks that fail the per-method model). Bare shape, no
        ``data`` block; not one of the five canonical categories. An
        unparseable body is rejected as HTTP 422 *before* this handler
        (we do not emit -32700); unknown method is folded here rather
        than -32601 (see Decision 006 §"What carries a JSON-RPC envelope").
-32602  validation   — input shape, length, file size, structural limit
        breach; see Decision 003 and Decision 006.
-32001  quota        — token or turn quota exhausted; carries
        ``data.resetAt``; see Decision 005 and Decision 006.
-32002  safety       — code-of-conduct or content-policy violation;
        see Decision 006.
-32003  rate_limit   — application-level concurrency or per-turn caps
        (``concurrent_sessions`` per-member session cap at initialize,
        ``concurrent_conversations``, ``tool_calls_per_turn``,
        ``task_agents_per_input``). Distinct from Cloud Armor
        transport-level rate limits.
-32603  internal     — unanticipated server failure (incl. the
        process/fleet-wide session backstop); assistant state
        transactionally reverts so the next ``message/send`` is safe.

Clients should code defensively against all five canonical codes so that a future enforcement-mode change requires no client-side update.