Enforce 90% code coverage threshold on PRs

[Copilot]

Coverage was measured but not enforced—PRs could merge regardless of coverage drops.

Changes

Modified .codecov.yml to block PRs when coverage falls below 90%:

• Project coverage: Changed from informational: true (report-only) to informational: false (blocking)
• Target: Set explicit 90% threshold instead of auto
• Threshold: Maintained 0% tolerance consistent with existing patch coverage enforcement

Both project (overall) and patch (changed lines) coverage checks now require ≥90% to pass.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[BUG] Low code coverage should block PR</issue_title>
<issue_description>## Expected Behavior
If code coverage for PR is <90%, PR should be blocked

Actual Behavior

Code coverages is measured, but not used.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [BUG] Low code coverage should block PR #750

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