Milestone 2: legacy compatibility processor #3462

Work:

• Implement one generic compatibility processor bound to one resolved logical agent.
• Forward raw request bytes, content type, headers, cookies, and cancellation to agent /run.
• Return raw status, repeated headers, cookies, and body.
• Do not parse or translate the agent contract.
• Do not add retries for non-idempotent /run.
• Generate one deterministic compatibility processor per resolved logical agent.
• Keep direct-agent routing as rollback until we’re ready to fully cut over to this design.

Definition of done:

• Every current taskset can run through a compatibility processor without changing its agent.
• Success, 4xx, 429, 5xx, invalid JSON, empty body, large error, repeated cookie, and cancellation parity pass.
• Rollout result, sidecar, retry, score, and aggregation output match direct-agent routing.