fix: verify null token_usage jq division in Copilot Token Optimizer

[Copilot]

The Copilot Token Optimizer workflow failed because map(.token_usage) | add returns null when all runs in a group have zero token usage (Go's omitempty omits the field), causing null / length to throw a fatal jq error.

Root cause

jq: error (at copilot-runs.json:12310): null (null) and number (1) cannot be divided

Single-run group with no token data: [null] | add → null, then null / 1 → error.

Fix (applied in PR #24400)

Added // 0 fallbacks in .github/workflows/copilot-token-optimizer.md:

- total_tokens: (map(.token_usage) | add),
- total_cost: (map(.estimated_cost) | add),
- avg_tokens: ((map(.token_usage) | add) / length),
+ total_tokens: (map(.token_usage // 0) | add),
+ total_cost: (map(.estimated_cost // 0) | add),
+ avg_tokens: ((map(.token_usage // 0) | add) / length),

null // 0 converts absent fields to 0, so [0] | add → 0 and 0 / 1 → 0. The lock file was recompiled and is consistent.