internal/jsonrpc2: reject fractional and out-of-range request IDs

[ez-lbz]

Summary

MakeID currently truncates any float64 request ID with int64(), so a request carrying {"id": 1.9, ...} is answered with {"id": 1, ...}. JSON-RPC 2.0 specifies that request IDs must be integers, strings, or null; silently rewriting a non-integer ID breaks response correlation for clients that match responses by ID, and can cause responses for distinct requests to be mixed up.

This change validates that float64 IDs are whole numbers within the int64 range before converting, and returns a parse error otherwise.

Behavior

Request ID	Before	After
1.9	accepted as 1	parse error
2.5	accepted as 2	parse error
9.3e18	truncated/overflow	parse error
42	accepted as 42	accepted as 42

Tests

• Added TestMakeIDRejectsNonInteger, TestMakeIDAcceptsInteger, and TestDecodeMessageRejectsFractionalID in internal/jsonrpc2/wire_test.go.