Conversations filter: local /v1/conversations endpoints

[leseb]

Parent

Part 2 of #602. Depends on #622.

Description

Implement the openai_conversations filter that handles all 8 /v1/conversations endpoints locally via FilterAction::Reject, backed by the store layer from #622.

Scope

• ConversationsConfig with self-contained database config (backend, database_url, table names, SSL)
• OpenaiConversationsFilter with lazy OnceCell store init, path matching, lifecycle dispatch
• 8 handler functions: create/get/update/delete conversation, create/list/get/delete items
• Metadata validation (max 16 keys, key ≤ 64 bytes, string values ≤ 512 bytes)
• Cursor-based pagination for item listing (after, limit 1–100, order asc/desc)
• Register openai_conversations in FilterRegistry
• Module exports through openai/mod.rs → ai/mod.rs → builtins/mod.rs
• Example config: examples/configs/ai/openai/conversations/conversations.yaml
• Integration tests covering all 8 endpoints + error cases
• cargo xtask sync-example-readme --fix

Acceptance Criteria

• All 8 endpoints return correct JSON shapes (conversation object, list, deleted, error)
• 404 for missing/wrong-tenant resources
• Requests never forwarded upstream
• make lint and make test-integration clean
• Example config parses and works end-to-end