Model JSON-RPC/LSP error codes as an enum instead of magic ints

[Firehed]

Summary

ResponseError (src/Protocol/ResponseError.php) hardcodes each JSON-RPC / LSP
error code as a magic integer inside a static factory (parseError -> -32700,
invalidRequest -> -32600, serverNotInitialized -> -32002, etc.). These codes
are a fixed, spec-defined set (JSON-RPC 2.0 + LSP 3.17), which is exactly what an
int-backed enum models: it names the numbers in one place and removes the inline
literals.

Proposed change

Introduce something like:

enum ErrorCode: int
{
    case ParseError = -32700;
    case InvalidRequest = -32600;
    case MethodNotFound = -32601;
    case InvalidParams = -32602;
    case InternalError = -32603;
    case ServerNotInitialized = -32002;
}

and have ResponseError carry it (adjusting jsonSerialize() to emit
$this->code->value).

Design decision to make

JSON-RPC reserves -32000..-32099 for server-defined codes and LSP permits custom
ones, so a pure enum cannot represent an arbitrary future code. Every code the
server emits today is standard, so an enum is sufficient now, but decide whether
ResponseError::$code becomes strictly the enum or ErrorCode|int before
committing to the shape.

Scope note

This is a refactor of a pre-existing pattern, deferred out of slice S1.4 (which
only added serverNotInitialized consistent with the existing factories) to keep
that slice's diff focused.

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This issue was written by AI (reviewed by a human).