fix: ALTER SNIPPET no longer fails with "page not found" (#402)

The AST visitor sets ContainerType to "SNIPPET" (uppercase) but execAlterPage
compared against lowercase "snippet", so the snippet branch was never taken
and the call always fell through to findPageByName, producing "page not found".

Fix: normalise ContainerType to lowercase via strings.ToLower before the
comparison. Added regression test with uppercase ContainerType to cover the
visitor-produced form.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ako

mdl/executor/cmd_alter_page.go

@@ -30,7 +30,7 @@ func execAlterPage(ctx *ExecContext, s *ast.AlterPageStmt) error {
 
 	var unitID model.ID
 	var containerID model.ID
-	containerType := s.ContainerType
+	containerType := strings.ToLower(s.ContainerType)
 	if containerType == "" {
 		containerType = "page"
 	}

mdl/executor/cmd_alter_page_mock_test.go

@@ -135,6 +135,38 @@ func TestAlterPage_Snippet_Success(t *testing.T) {
 	assertContainsStr(t, buf.String(), "Altered snippet")
 }
 
+// Issue #402 — visitor sets ContainerType to uppercase "SNIPPET"; executor
+// must normalise before comparing so the snippet branch is taken.
+func TestAlterPage_Snippet_UppercaseContainerType_Issue402(t *testing.T) {
+	mod := mkModule("MyModule")
+	snp := mkSnippet(mod.ID, "TestSnippet")
+	saved := false
+	mb := &mock.MockBackend{
+		IsConnectedFunc: func() bool { return true },
+		ListModulesFunc: func() ([]*model.Module, error) { return []*model.Module{mod}, nil },
+		ListFoldersFunc: func() ([]*types.FolderInfo, error) { return nil, nil },
+		ListSnippetsFunc: func() ([]*pages.Snippet, error) {
+			return []*pages.Snippet{snp}, nil
+		},
+		OpenPageForMutationFunc: func(unitID model.ID) (backend.PageMutator, error) {
+			return &mock.MockPageMutator{
+				SaveFunc: func() error { saved = true; return nil },
+			}, nil
+		},
+	}
+	h := mkHierarchy(mod)
+	withContainer(h, snp.ContainerID, mod.ID)
+	ctx, buf := newMockCtx(t, withBackend(mb), withHierarchy(h))
+	assertNoError(t, execAlterPage(ctx, &ast.AlterPageStmt{
+		ContainerType: "SNIPPET", // uppercase as produced by the AST visitor
+		PageName:      ast.QualifiedName{Module: "MyModule", Name: "TestSnippet"},
+	}))
+	if !saved {
+		t.Error("expected Save to be called")
+	}
+	assertContainsStr(t, buf.String(), "Altered snippet")
+}
+
 // ---------------------------------------------------------------------------
 // Open mutator error
 // ---------------------------------------------------------------------------