tree: Fix bug with rebasing over conflicted change (microsoft#22372)

## Description

This fixes a bug where ModularChangeFamily's rebase method was ignoring
that the base changeset is conflicted.

packages/dds/tree/src/feature-libraries/modular-schema/modularChangeFamily.ts

@@ -673,7 +673,7 @@ export class ModularChangeFamily
 			0x89a /* Unexpected destroys in change to invert */,
 		);
 
-		if ((change.change.constraintViolationCount ?? 0) > 0) {
+		if (hasConflicts(change.change)) {
 			return makeModularChangeset(
 				undefined,
 				undefined,
@@ -827,6 +827,10 @@ export class ModularChangeFamily
 		over: TaggedChange<ModularChangeset>,
 		revisionMetadata: RevisionMetadataSource,
 	): ModularChangeset {
+		if (hasConflicts(over.change)) {
+			return taggedChange.change;
+		}
+
 		const change = taggedChange.change;
 		const maxId = Math.max(change.maxId ?? -1, over.change.maxId ?? -1);
 		const idState: IdAllocationState = { maxId };
@@ -1845,7 +1849,7 @@ export function intoDelta(
 	const idAllocator = MemoizedIdRangeAllocator.fromNextId();
 	const rootDelta: Mutable<DeltaRoot> = {};
 
-	if ((change.constraintViolationCount ?? 0) === 0) {
+	if (!hasConflicts(change)) {
 		// If there are no constraint violations, then tree changes apply.
 		const fieldDeltas = intoDeltaImpl(
 			change.fieldChanges,

packages/dds/tree/src/test/shared-tree/editing.spec.ts

@@ -3124,6 +3124,40 @@ describe("Editing", () => {
 				expectJsonTree([tree, tree2], [{}]);
 			});
 		});
+
+		it("Rebase over conflicted change", () => {
+			const tree1 = makeTreeFromJsonSequence(["A", "B"]);
+			const tree2 = tree1.fork();
+
+			// Remove A
+			remove(tree1, 0, 1);
+
+			// This transaction will be conflicted after rebasing since the previous edit deletes the constrained node.
+			tree2.transaction.start();
+			tree2.editor.addNodeExistsConstraint({
+				parent: undefined,
+				parentField: rootFieldKey,
+				parentIndex: 0,
+			});
+
+			// Remove B
+			remove(tree2, 1, 1);
+			tree2.transaction.commit();
+
+			const tree3 = tree1.fork();
+
+			// This edit will be rebased over the conflicted transaction.
+			insert(tree3, 1, "C");
+
+			tree1.merge(tree2, false);
+			tree1.merge(tree3, false);
+
+			tree2.rebaseOnto(tree1);
+			tree3.rebaseOnto(tree1);
+
+			const expected = ["B", "C"];
+			expectJsonTree([tree1, tree2, tree3], expected);
+		});
 	});
 
 	it.skip("edit removed content", () => {