Add env-based multi-session round-robin routing for v1/v2/v3 completions

[cortensor-ryuma]

• add a shared completion session resolver backed by ROUTER_COMPLETION_SESSION_IDS
• support router-side round-robin session selection when session_id is omitted
• preserve existing precedence so explicit path or payload session_id still wins
• wire the resolver into v1, v2, and v3 completion flows
• ensure v2 and v3 resolve session before offchain/encryption-aware payload preparation
• keep x402, delegate, validate, and other non-completion routing paths unchanged