fix: handle wrapped x402 premium payment failures with safer fallback

[tsubasakong]

Summary

• treat wrapped x402 merchant-proxy payment failures as provider errors that can fallback
• improve session/profile fallback routing to avoid sticky premium routing after paid-model failure
• add regression tests for wrapped x402 error fallback and profile re-routing

Test Plan

• bun run test/premium-compat.ts
• npm run typecheck

[VickyXAI]

Thanks — closing the diff, but the underlying question is worth keeping.

Stale: main's setSession() 4th param is now userExplicit: boolean, not routingProfile, so this would pass a profile string into the explicit-pin slot. SessionEntry has since grown three-strike escalation and sessionCostMicros.

Also, one part I'd push back on regardless of age: adding /payment required/i and /merchant_status\"?\\s*:\\s*402/i to PROVIDER_ERROR_PATTERNS reclassifies "your wallet is out of money" as "the provider is flaky", and /payment required/i is broad enough to match model output text. Main deliberately separates these — it has dedicated invalid_payload and insufficient_funds branches with specific guidance — so this would mask real payment failures behind a silent reroute.

The bug you found may well still be live though: if a premium → eco profile switch still leaves a session pinned to the paid model, please open an issue with a repro and we'll fix it against the current session code.