[M1] Add A2A JSON-RPC envelope parser and validator processor

[rdobrik]

Description

Implement a processor equivalent to MCP envelope handling that:

• Parses JSON-RPC 2.0 payloads
• Validates required fields
• Distinguishes request/notification/response semantics
• Extracts method and id into exchange properties
• Normalizes params for downstream processors

Tasks

• Add A2AJsonRpcEnvelopeProcessor.
• Define exchange property names for type/id/method/raw payload.
• Validate jsonrpc == "2.0" and payload object shape.
• Add method whitelist support for current phase.

Acceptance Criteria

• Invalid envelopes return deterministic errors.
• Valid envelopes set exchange properties consistently.
• Unit tests cover malformed JSON, missing fields, and happy path.

Dependencies

• Issue 1