[M1] Implement standardized JSON-RPC error processor for A2A

[rdobrik]

Description

Create a reusable error processor that emits JSON-RPC 2.0 compliant error envelopes and maps validation/execution failures to correct error codes.

Tasks

• Add A2AErrorProcessor.
• Define error code mapping strategy (-32600, -32601, -32602, -32603).
• Ensure ID echoing behavior follows JSON-RPC rules.
• Replace direct/manual error JSON creation in routes/processors.

Acceptance Criteria

• Error envelopes are spec-conformant.
• Error messages are deterministic and test-covered.
• Route-level onException uses shared error processor.

Dependencies

• Issue 2