refactor(#2796): shared JsonRpc helper for MCP — wire constants, erro…

[Skobeltsyn]

…r codes, McpException hierarchy

New agents_engine.mcp.JsonRpc consolidates the JSON-RPC 2.0 envelope shape that was hand-rolled separately on both sides:

JsonRpc helper

• encodeRequest(id, method, params) — builds request/notification envelopes; passing null for id signals a notification.
• encodeResult(id, result) / encodeError(id, code, message) — response envelope builders the server now uses.
• parseEnvelope(payload) — single Map lookup point.
• isNotification(envelope) — the duplicated !containsKey("id") || method.startsWith("notifications/") predicate, in one place.

Wire constants + error codes

• JsonRpcWire centralises the literal "2.0", the key names (jsonrpc/method/params/id/result/error/code/message), and the notification prefix.
• JsonRpcErrorCode names the standard JSON-RPC error codes (-32700/-32600/-32601/-32602/-32603) as PARSE_ERROR / INVALID_REQUEST / METHOD_NOT_FOUND / INVALID_PARAMS / INTERNAL_ERROR. Every bare literal in McpServer.kt replaced.

McpException hierarchy

• sealed class McpException : IllegalStateException (extends ISE so existing catch (e: IllegalStateException) test sites keep working — no breaking change for downstream consumers).
• Three subclasses: Transport (network / stdio EOF), Protocol (JSON-RPC error response or malformed envelope), ToolFailure (tool ran but signalled error). McpClient.post now throws McpException.Protocol instead of bare error(), and the helper's parseResponse does the same.

Side effect (small)

• McpClient's handshake() notifications/initialized raw-string emission now flows through JsonRpc.encodeRequest so the wire is identical to every other notification.

Full ./gradlew test green; 254+ TEST-*.xml files, zero failures/errors.