Raise Test Coverage Improver daily AI credit guardrail (#7890)

The `Test Coverage Improver` workflow was being blocked before agent
execution because its 24-hour effective AI credit usage exceeded the
repository default daily threshold (`7.7K` vs `5K`). This change gives
the workflow an explicit per-workflow daily budget aligned with its
scheduled coverage-analysis workload.

- **Workflow budget override**
- Adds an explicit `max-daily-ai-credits: 10K` to
`test-coverage-improver.md`
- Stops this workflow from inheriting the stricter default `5K` daily
guardrail

- **Compiled workflow update**
- Regenerates `test-coverage-improver.lock.yml` so the activation job
exports the updated `GH_AW_MAX_DAILY_AI_CREDITS=10000`
- Keeps runtime guardrail enforcement consistent with the markdown
source

- **Net effect**
  - Preserves the existing workflow behavior and schedule
  - Reduces skipped runs caused solely by the default daily AIC cap

```yaml
engine: copilot
max-daily-ai-credits: 10K
```

Author: lpcox