chore(scripts): validate react-native export conditions in constraints (#4370)

Add a yarn constraint for published packages:
- each exports entry's "react-native" condition must point to a file
  that exists (auto-fixed when only the .ts/.tsx extension is wrong)
- packages that peer-depend on react-native must provide a
  "react-native" condition for every export

This catches @fluentui-react-native/tester, whose "react-native"
condition pointed at ./src/index.ts while the entry is ./src/index.tsx;
fixed via yarn constraints --fix.

Part of #3936

Author: UditDewan

.changeset/tester-react-native-export.md

@@ -0,0 +1,5 @@
+---
+'@fluentui-react-native/tester': patch
+---
+
+Point the `react-native` export condition at `./src/index.tsx`; it referenced `./src/index.ts`, which does not exist.

apps/fluent-tester/package.json

@@ -16,7 +16,7 @@
   "exports": {
     ".": {
       "types": "./lib/index.d.ts",
-      "react-native": "./src/index.ts",
+      "react-native": "./src/index.tsx",
       "import": "./lib/index.js",
       "default": "./src/index.tsx"
     }

scripts/src/tasks/constrain.ts

@@ -82,6 +82,7 @@ export function constrain(workspace: Yarn.Constraints.Workspace): void {
   enforceBuildInfoLocation(ctx);
   assertDeprecations(ctx);
   useCatalogs(ctx);
+  ensureReactNativeExports(ctx);
 }
 
 export function constrainRoot(workspace: Yarn.Constraints.Workspace): void {
@@ -118,6 +119,36 @@ function useCatalogs(ctx: PackageContext): void {
   }
 }
 
+/**
+ * Metro resolves the "react-native" export condition to TypeScript source. Ensure published packages point that condition
+ * at a file that exists, and that React Native libraries provide it for every export.
+ */
+function ensureReactNativeExports(ctx: PackageContext): void {
+  const { exports, peerDependencies, private: isPrivate } = ctx.manifest;
+  if (isPrivate || !exports || typeof exports !== 'object' || Array.isArray(exports)) {
+    return;
+  }
+  const requireCondition = Boolean(peerDependencies?.['react-native']);
+  for (const [subpath, conditions] of Object.entries(exports)) {
+    if (!conditions || typeof conditions !== 'object' || Array.isArray(conditions)) {
+      continue;
+    }
+    const target = (conditions as Record<string, unknown>)['react-native'];
+    if (typeof target === 'string') {
+      if (!repoCtx.hasFile(ctx.root, target)) {
+        const swapped = target.endsWith('.tsx') ? target.slice(0, -1) : target.endsWith('.ts') ? `${target}x` : undefined;
+        if (swapped && repoCtx.hasFile(ctx.root, swapped)) {
+          ctx.enforce(['exports', subpath, 'react-native'], swapped);
+        } else {
+          ctx.error(`exports["${subpath}"]["react-native"] points to ${target}, which does not exist`);
+        }
+      }
+    } else if (requireCondition) {
+      ctx.error(`exports["${subpath}"] is missing a "react-native" condition pointing to its TypeScript source`);
+    }
+  }
+}
+
 function assertDeprecations(ctx: PackageContext): void {
   // placeholder to add deprecated packages or parts of package.json to remove them
   ctx.enforce('scripts.lint-package', undefined);