ci: add coverage threshold gate (40% minimum)

[ArangoGutierrez]

Summary

• Adds coverage threshold check (40% minimum) after make coverage in CI
• Uses go tool cover -func to extract total coverage, fails with ::error:: annotation if below threshold
• Conservative initial threshold since PR build: expand test coverage scope to cmd/, api/, and internal/ #750 just expanded coverage scope to include cmd/, api/, internal/
• Threshold can be raised incrementally as coverage improves

Test plan

• Verify YAML syntax is correct
• Confirm step runs after coverage generation and before Coveralls upload
• Verify bc is available on ubuntu-latest runners

[coveralls]

Pull Request Test Coverage Report for Build 23744430291

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 45.802%

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Totals
Change from base Build 23740406589:	0.0%
Covered Lines:	4975
Relevant Lines:	10862

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💛 - Coveralls