AGENTS.md: require translating domain-layer exceptions to HTTPException at FastAPI route boundaries

[jason810496]

• related: AIP-103: Add Core API endpoints for task state and asset state #67041 (comment), Apply per-DAG audit log permission to event log detail endpoint #67112

Why

Core API route handlers that let domain-layer ValueErrors (e.g., from MetastoreStateBackend) bubble up surface as 500 Internal Server Error, leaking internals and misleading clients. Capture the rule so future agents catch and re-raise as HTTPException with the right status.

What

• Add one bullet under ## Coding Standards in root AGENTS.md directing agents to catch domain-layer exceptions in core_api route handlers and re-raise as HTTPException (404 not-found, 400 invalid input).

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Was generative AI tooling used to co-author this PR?

• Yes, with help of Claude Code (Opus 4.7) following the guidelines