[rush] Route lockfile-changed warning to stderr (#5804)

`rush list --json` writes `Lockfile was created or deleted. Assuming
all projects are affected.` to stdout when the lockfile was added or
removed in the diff range, which corrupts the JSON output and breaks
pipes such as `rush list --json --impacted-by git:<sha> | jq .`.

The information is useful diagnostic context, so rather than suppress
it, route it through `terminal.writeWarningLine` (which writes to
stderr) -- the standard CLI convention for warnings that should not
pollute machine-readable stdout. Matches how the rest of this file
already emits non-fatal diagnostics (see lines 418, 446, 458).

Fixes #5406

Author: LeSingh1

common/changes/@microsoft/rush/fix-rush-list-json-stderr_2026-05-17-19-00.json

@@ -0,0 +1,11 @@
+{
+  "changes": [
+    {
+      "comment": "Route the \"Lockfile was created or deleted\" warning to stderr so that machine-readable output (e.g. `rush list --json`) remains parseable when the lockfile was added or removed in the diff range.",
+      "type": "patch",
+      "packageName": "@microsoft/rush"
+    }
+  ],
+  "packageName": "@microsoft/rush",
+  "email": "sshaurya914@gmail.com"
+}

libraries/rush-lib/src/logic/ProjectChangeAnalyzer.ts

@@ -220,11 +220,13 @@ export class ProjectChangeAnalyzer {
         if (shrinkwrapStatus) {
           if (shrinkwrapStatus.status !== 'M') {
             if (rushConfiguration.subspacesFeatureEnabled) {
-              terminal.writeLine(
+              terminal.writeWarningLine(
                 `"${subspace.subspaceName}" subspace lockfile was created or deleted. Assuming all projects are affected.`
               );
             } else {
-              terminal.writeLine(`Lockfile was created or deleted. Assuming all projects are affected.`);
+              terminal.writeWarningLine(
+                `Lockfile was created or deleted. Assuming all projects are affected.`
+              );
             }
             for (const project of subspaceProjects) {
               changedProjects.add(project);