ci: skip coverage checks without fresh flag uploads

[thorstendb-ARM]

Summary

• Exclude component coverage checks when no fresh flag report was uploaded for the commit, including carried-forward reports.
• Keep carryforward data, coverage targets, thresholds, and upload failure handling unchanged.
• Prevent false cross-component regressions such as the SVDConv check in feat(ctrace): process all trace channels per solution set #2633 after merging test(svdconv): expand unit and generator coverage #2634.

Validation

• Official Codecov YAML validator: Valid!.
• git diff --check passed. Only codecov.yml changes; no source or workflow changes.

[thorstendb-ARM]

This change fixes the false alarm, but does not guarantee a minimum overall coverage level for devtools. Under that requirement, this PR does not provide a complete solution on its own.

Why did SVDConv fail despite the coverage improvement?

The improvement was real:

• test(svdconv): expand unit and generator coverage #2634: SVDConv coverage increased from 48.03% to 60.31%.
• ctrace PR feat(ctrace): process all trace channels per solution set #2633: The path filters did not trigger another SVDConv run. Codecov carried forward an old report showing 48.03%.
• Codecov compared that report with the updated main at 60.31%, producing an apparent regression.

The additional tests were present. The measurement data was outdated, not the code.

What does this change affect?

For all six components, the existing coverage requirements remain unchanged when a fresh report is uploaded.

Without a fresh report, the corresponding status check is omitted. This prevents the false alarm described above, but has a limitation: an unexpectedly missing report is also excluded. Codecov does not distinguish between an intentionally skipped test run and a missing upload. See the Codecov documentation.

What is still needed to protect overall coverage?

Even before this PR, the overall status check was disabled with project.default: false. The component checks compare coverage against the base and each allow a decrease of 0.1 percentage points; no fixed overall minimum is configured. See codecov.yml.

To enforce an overall coverage requirement, we also need:

• A required overall coverage check with a defined minimum.
• Complete, consistent coverage data.
• A failure when an expected report is missing.

Simply enabling the overall check would reproduce the same false alarm if it still uses outdated carried-forward reports.

The remaining policy decision is: Should overall coverage stay above a fixed percentage (and if so, which one), or must it never decrease compared with main? The complete solution depends on that choice.