reject proof requests early for verifier-only instances

[chetanyb]

Summary

Verifier-only zkVM instances (kind: verifier) can only verify proofs received via gossip - they cannot generate proofs. Previously, proof generation requests to verifier-only instances were:

1. Accepted by the HTTP handler
2. Witness fetched from EL (expensive and wasteful)
3. Failed silently at the worker dispatch layer (no worker exists)

This PR rejects proof requests early at the HTTP layer with a 400 Bad Request, before any witness fetching occurs.

Changes

• Early rejection: Return HTTP 400 for proof generation requests targeting verifier-only zkvms
• Startup logging: Add mode field (prover/mock/verifier-only) to zkvm instance creation log
• Verifier-only info: Log at startup that proof generation requests will be rejected
• Debug logging: Log each rejected request at debug level

Behavior

Startup log

INFO zkboost_server::server: zkvm instance created proof_type=ethrex-zisk mode="verifier-only"
INFO zkboost_server::server: verifier-only mode: proof generation requests will be rejected proof_type=ethrex-zisk

Per-request (debug level)

DEBUG post_execution_proof_requests: zkboost_server::http::v1::post_execution_proof_requests: rejecting proof request: verifier-only instance proof_type=ethrex-zisk

[han0110]

LGTM!