[Refactoring]: Refactor version header validation into a shared validator

[MisterVVP]

Summary

Move ValidateVersionHeader out of the REST and JSON-RPC transport implementations into a dedicated validator class. Transports should own the validator via composition and invoke it when validating incoming requests.

Motivation

REST and JSON-RPC transports currently carry transport-local version header validation logic. As more transport-level validation has moved behind shared validator classes, version validation should follow the same pattern to keep transport classes focused on routing, parsing, and response mapping.

Proposed Change

• Add a server-side validator class, for example VersionHeaderValidator.
• Store the required version-header policy in the validator constructor.
• Replace RestServerTransport::ValidateVersionHeader and JsonRpcServerTransport::ValidateVersionHeader with direct calls to the composed validator.
• Keep transport-specific error response formatting in the transport classes.
• Add or update focused unit/functional coverage for:
  • missing A2A-Version when required,
  • missing A2A-Version when optional,
  • unsupported A2A-Version,
  • accepted supported version.

Acceptance Criteria

• REST and JSON-RPC transports no longer define private ValidateVersionHeader member functions.
• Both transports compose and invoke the shared version validator.
• Existing version validation behavior and protocol error mapping remain unchanged.
• Formatting, unit tests, and clang-tidy pass.