Enable Rego tests on Windows now that Regal fixed upstream path handling (#1114)

- Bump Regal from v0.36.1 to v0.39.0 which includes Windows path fixes
  (upstream fix: https://github.com/StyraInc/regal/issues/1683)
- Remove sys.platform == "win32" skipif from TestRegoLanguageServer

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/pytest.yml

@@ -397,7 +397,7 @@ jobs:
       - name: Install Regal (Rego Language Server)
         shell: bash
         run: |
-          REGAL_VERSION="0.36.1"
+          REGAL_VERSION="0.39.0"
 
           if [[ "${{ runner.os }}" == "Linux" ]]; then
             if [[ "$(uname -m)" == "x86_64" ]]; then