chore(api,ci,docs,test): private-service schema, proto-check, fixups

Non-functional cleanups and contract/CI hardening around the
private-service work:

API schema (openapi.yml):
- Require a non-empty access_groups and mode=http when private=true,
  on both Service and ServiceRequest, mirroring
  validatePrivateRequirements. mode stays optional-but-constrained
  (empty defaults to http server-side), matching runtime.

CI (proto-version-check.yml):
- Cover renamed .pb.go files (read base via previous_filename).
- Match protoc-gen-go-grpc version headers (optional "- " prefix and
  -gen-go-grpc suffix) so grpc-generated files are in scope.

Docs / comments:
- Reword Config field docs to say defaults are applied at Server.Start
  (initDefaults), not New.
- Rename the obsolete --private-inbound flag to --private across
  comments and the proto doc.

Pre-existing test fixups surfaced by review:
- Repair the integration-tagged validate_session_test.go (SignToken
  signature growth + new Manager interface methods).
- Fix the CI-skip boolean precedence so Windows isn't skipped
  unconditionally.
- Guard the router.HTTPListener type assertion with comma-ok.

management/internals/shared/grpc/validate_session_test.go

@@ -102,7 +102,7 @@ func generateSessionKeyPair(t *testing.T) (string, string) {
 
 func createSessionToken(t *testing.T, privKeyB64, userID, domain string) string {
 	t.Helper()
-	token, err := sessionkey.SignToken(privKeyB64, userID, domain, auth.MethodOIDC, nil, time.Hour)
+	token, err := sessionkey.SignToken(privKeyB64, userID, "", domain, auth.MethodOIDC, nil, nil, time.Hour)
 	require.NoError(t, err)
 	return token
 }
@@ -394,6 +394,10 @@ func (m *testValidateSessionProxyManager) ClusterSupportsCrowdSec(_ context.Cont
 	return nil
 }
 
+func (m *testValidateSessionProxyManager) ClusterSupportsPrivate(_ context.Context, _ string) *bool {
+	return nil
+}
+
 type testValidateSessionUsersManager struct {
 	store store.Store
 }
@@ -401,3 +405,24 @@ type testValidateSessionUsersManager struct {
 func (m *testValidateSessionUsersManager) GetUser(ctx context.Context, userID string) (*types.User, error) {
 	return m.store.GetUserByUserID(ctx, store.LockingStrengthNone, userID)
 }
+
+func (m *testValidateSessionUsersManager) GetUserWithGroups(ctx context.Context, userID string) (*types.User, []*types.Group, error) {
+	user, err := m.store.GetUserByUserID(ctx, store.LockingStrengthNone, userID)
+	if err != nil {
+		return nil, nil, err
+	}
+	if len(user.AutoGroups) == 0 {
+		return user, nil, nil
+	}
+	groupsMap, err := m.store.GetGroupsByIDs(ctx, store.LockingStrengthNone, user.AccountID, user.AutoGroups)
+	if err != nil {
+		return nil, nil, err
+	}
+	groups := make([]*types.Group, 0, len(user.AutoGroups))
+	for _, id := range user.AutoGroups {
+		if g, ok := groupsMap[id]; ok && g != nil {
+			groups = append(groups, g)
+		}
+	}
+	return user, groups, nil
+}