Accept any string as a request id

[dominikklein]

MCP allows any string as a request id, but MCP::Server#handle and #handle_json called JsonRpcHandler with its default pattern, which only lets letters, digits, _ and - through. They now pass id_validation_pattern: nil, so ids like req.1, call:42 or a URN work. JsonRpcHandler keeps its default for anyone calling it directly.

server.handle_json('{"jsonrpc":"2.0","id":"req.1","method":"tools/list"}')
# => {"jsonrpc":"2.0","id":"req.1","result":{"tools":[]}}

If the restriction was meant to keep odd characters out of logs, an option on the server would work as well; happy to switch to that.

Fixes #603.

[koic]

Closing in favor of #613, which makes the same change to handle and handle_json and adds a guard: without the pattern, a string id that is not valid UTF-8 passes valid_id?, the method runs, and the response cannot be encoded, a 500 on
the modern path and the end of the stdio loop. valid_id? now refuses such a string whether or not a pattern is given.

#613 also adds regression tests for that case, for ids holding a newline, markup, spaces and the empty string, for an error response echoing such an id, and for the types still refused.

Thank you for the report and the pointer to the id_validation_pattern: keyword in #603.