Add CI test stage with coverage reporting and test infrastructure

- Updated .github/workflows/ci.yml to include:
  - Go module caching for faster builds
  - Coverage report generation and upload to Codecov
  - 80% coverage threshold check
- Created tests/ directory with integration test framework
- Added test fixtures and configuration
- Initial integration test passes

Related: FRE-8b42289c (Pop: Add CI test stage to workflow)

.github/workflows/ci.yml

@@ -21,14 +21,45 @@ jobs:
       with:
         go-version: ${{ matrix.go-version }}
 
+    - name: Cache Go modules
+      uses: actions/cache@v4
+      with:
+        path: |
+          ~/.cache/go-build
+          ~/go/pkg/mod
+        key: ${{ runner.os }}-go-${{ matrix.go-version }}-${{ hashFiles('**/go.sum') }}
+        restore-keys: |
+          ${{ runner.os }}-go-${{ matrix.go-version }}-
+
     - name: Download dependencies
       run: go mod download
 
     - name: Build
       run: go build -v ./...
 
-    - name: Test
-      run: go test -v -race ./...
+    - name: Test with coverage
+      run: go test -v -race -coverprofile=coverage.out -covermode=atomic ./...
+
+    - name: Calculate coverage
+      run: |
+        TOTAL=$(go test -cover ./... 2>&1 | grep -oP '\d+\.\d+%$' | head -1 | tr -d '%')
+        echo "Coverage: ${TOTAL}"
+        if [ -z "$TOTAL" ]; then
+          echo "No coverage data found"
+          exit 1
+        fi
+        if (( $(echo "$TOTAL < 80" | bc -l) )); then
+          echo "Coverage ${TOTAL}% is below 80% threshold"
+          exit 1
+        fi
+        echo "Coverage ${TOTAL}% meets 80% threshold"
+
+    - name: Upload coverage report
+      uses: codecov/codecov-action@v4
+      with:
+        files: ./coverage.out
+        flags: unittests
+        name: codecov-pop
 
     - name: Lint
       run: |