fix(mcp): preserve JSON-RPC string ids instead of coercing to int (#253)

JSON-RPC 2.0 §4 permits string request ids, and several MCP clients
(including Claude Desktop) send a string id for "initialize". The parser
coerced string ids via strtol, yielding 0 for any non-numeric id, so the
response echoed the wrong id and the client could not correlate it.

Carry the string id verbatim through parse -> dispatch -> response
(new id_str field on the request/response structs) and emit it as a JSON
string. Numeric ids are unchanged. The method-not-found error path now
echoes the original id too.