Add code coverage reporting to CI

[cursor]

Problem

coverlet.collector is referenced in DBToolsUnitTest.csproj, but CI does not collect or publish coverage reports. There is no coverage threshold or visibility into untested areas.

Proposed solution

1. Update .github/workflows/ci.yml to collect coverage:

   dotnet test --collect:"XPlat Code Coverage"

2. Upload results to GitHub Actions artifacts (or Codecov/Coveralls if desired)
3. Optionally add coverlet.runsettings to exclude generated/boilerplate code
4. Document how to view coverage locally

Acceptance criteria

• CI produces a coverage report artifact on each run
• README or contributing guide explains how to run coverage locally
• Optional: minimum coverage gate for PRs (can be a follow-up)

Related

Complements #28 (new tests) and PR #24 (CI foundation).