Skip LLM execution analysis for credit exhaustion failures

Add early-exit in generate_activity_status_for_execution() that detects
credit/balance exhaustion errors and returns a static ActivityStatusResponse
without making an LLM call.

48% of all execution failures (1,472/3,048 in the last 24h) are credit
exhaustion, each triggering a paid LLM call to conclude 'no credits.'
This skips the LLM entirely for these known failures, saving ~$1-2/day.

Co-authored-by: Bently <[email protected]>