observe: failed executions lose recorded USD when token usage is absent #142

Summary

After retaining provider prices on failed terminal events (#140), the cost reader still discards a failed execution’s real USD when its token total is zero or absent. A provider can supply a price without token usage; known dollars must not depend on a separate optional measurement.

Reproduction

Use a scripted model that returns llm_output={"token_usage": {"cost": 0.002}} but no message usage_metadata; invoke it in a node and then raise. The terminal trace event has cost_usd=0.002, tokens=0, while attribute(trace, run_id).recorded_cost_usd is None. replay(...).recorded_cost_usd retains the price, so the two readers disagree.

Cause

grapharc/observe/cost.py:_price_run handles a failed node’s price only inside if execution.tokens:. Successful node terminals already preserve prices without requiring tokens.

Fix boundary

Process an explicitly recorded failure price even when tokens are zero. Keep absent prices absent and estimates reader-side; retain failed-agent subcall prices as a breakdown rather than adding them again.

Verification

Cover absent, zero and positive prices on success/error events; include a real scripted failed node without token metadata and run/thread/tenant attribution. Run the replay/cost and full suites with paid tests excluded.

Acceptance

• Known provider USD survives run/thread/tenant attribution without token metadata.
• A zero price is recorded zero; a missing price stays missing.
• Replay and cost attribution agree, without counting subcall prices twice.