feat(vercel_ai): add VAI-010 mutating tool idempotency rule

[joeymussalli]

Summary

• Add vercel_ai/idempotency.yaml with VAI-010: TypeScript mutating Vercel AI tools (create/send/charge/…) with no idempotency marker in the tool definition.
• Vercel leaves ToolDef.Name empty (model-facing name is the agent's tools-record key), so matching relies on VarName via name_has_prefix — pairs with the engine PR that falls back to VarName when Name is empty.

Stack

1. Engine: VarName fallback + fixture + fire/silent cases
2. This PR — rule YAML
3. Rulebook: rationale doc

Test plan

• Engine CI green with paired engine branch (or merge engine first)
• trustabl rules validate over the packs
• Confirm no ID collision with open VAI-013..015 PRs

Made with Cursor