chore: optimize test-coverage-improver workflow for ~50% token reduction (#2238)

* Initial plan

* chore: optimize test-coverage-improver workflow for token efficiency

Agent-Logs-Url: https://github.com/github/gh-aw-firewall/sessions/37fb82aa-b951-4d52-89ab-d7e3f355d8ad

* fix: use grouped heredoc redirection to correctly write step outputs to GITHUB_OUTPUT

Agent-Logs-Url: https://github.com/github/gh-aw-firewall/sessions/933aab55-648e-4382-a725-82fd63674887

Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

Author: Copilot