ci: gate Core coverage and exclude test tooling from the report

Coverage settings exclude the fuzz harness, server probe and test assembly;
CI fails if Core drops below 95% lines or 88% branches. The gitignore no
longer swallows tests/coverage.runsettings.

.gitignore

@@ -24,6 +24,7 @@ packages/
 TestResults/
 coverage/
 coverage.*
+!tests/coverage.runsettings
 *.coverage
 *.coveragexml
 *.trx