[codex] fix state-pg setIfNotExists TTL expiry

[Zygimantass]

Summary

Fixes the Postgres state adapter so setIfNotExists() can claim a cache key whose existing row has expired.

Previously the method used ON CONFLICT DO NOTHING, so an expired row in chat_state_cache still blocked acquisition until a separate cleanup deleted it. That diverged from the memory adapter behavior and from the expected lease semantics for cache-backed coordination.

The new query keeps active rows protected, but replaces rows whose expires_at is in the past.

Validation

• pnpm --filter @chat-adapter/state-pg test
• pnpm --filter @chat-adapter/state-pg typecheck
• pnpm exec ultracite check packages/state-pg/src/index.ts packages/state-pg/src/index.test.ts

[vercel]

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