fix: exclude main packages and infrastructure from coverage calculation

Packages like cmd/api, cmd/seed, internal/logger, and internal/metrics
are entrypoints and infrastructure code that don't benefit from unit
tests. These were being counted as 0% coverage in CI (which has the
full Go toolchain including covdata) but excluded locally (due to
'no such tool covdata' error), causing a ~2.5% coverage discrepancy.

Standard Go practice is to exclude such packages from coverage
calculations. This fix filters them from the coverage profile before
computing the total.

scripts/go-test-coverage.sh

@@ -10,6 +10,15 @@ MIN_COVERAGE="${CHARON_MIN_COVERAGE:-${CPM_MIN_COVERAGE:-85}}"
 
 cd "$BACKEND_DIR"
 
+# Packages to exclude from coverage (main packages and infrastructure code)
+# These are entrypoints and initialization code that don't benefit from unit tests
+EXCLUDE_PACKAGES=(
+    "github.com/Wikid82/charon/backend/cmd/api"
+    "github.com/Wikid82/charon/backend/cmd/seed"
+    "github.com/Wikid82/charon/backend/internal/logger"
+    "github.com/Wikid82/charon/backend/internal/metrics"
+)
+
 # Try to run tests to produce coverage file; some toolchains may return a non-zero
 # exit if certain coverage tooling is unavailable (e.g. covdata) while still
 # producing a usable coverage file. Don't fail immediately — allow the script
@@ -19,6 +28,16 @@ if ! go test -race -v -mod=readonly -coverprofile="$COVERAGE_FILE" ./...; then
     echo "Warning: go test returned non-zero; checking coverage file presence"
 fi
 
+# Filter out excluded packages from coverage file
+if [ -f "$COVERAGE_FILE" ]; then
+    FILTERED_COVERAGE="${COVERAGE_FILE}.filtered"
+    cp "$COVERAGE_FILE" "$FILTERED_COVERAGE"
+    for pkg in "${EXCLUDE_PACKAGES[@]}"; do
+        sed -i "\|^${pkg}|d" "$FILTERED_COVERAGE"
+    done
+    mv "$FILTERED_COVERAGE" "$COVERAGE_FILE"
+fi
+
 if [ ! -f "$COVERAGE_FILE" ]; then
     echo "Error: coverage file not generated by go test"
     exit 1