[Feature] Add JsonRpcMethodNameValidator for reusable method name policy enforcement

[limehee]

Problem statement

Method name policy checks (blank names, reserved namespace constraints such as rpc.*, and optional naming conventions) are currently spread across components. This increases duplication and risks inconsistent enforcement.

Proposed solution

Introduce JsonRpcMethodNameValidator in jsonrpc-core as a reusable policy utility:

• validate blank/whitespace names
• enforce reserved namespace policy (rpc.*)
• optional extension point for additional naming constraints (pattern/length/custom predicates)

Then reuse the same validator in request validation and method registration paths.

Alternatives considered

• Keep checks local in each component
• Add utility methods without a dedicated validator abstraction

These alternatives still make policy evolution harder and can reintroduce drift.

JSON-RPC behavior impact

No default protocol behavior change (current reserved namespace rejection remains). This is mainly an internal consistency and reuse improvement.

Additional context

• Keep default validator RFC-aligned and conservative.
• Ensure error mapping remains unchanged for invalid method names.
• Add tests proving consistent behavior across request validator and registration flows.