Skip third-party components whose class is not linked instead of crashing at launch (#59067)

Summary:
`RCTThirdPartyComponentsProvider.mm` builds its component map from a dictionary literal of `NSClassFromString` results. When a component's library is not linked into the target, such as an app extension or App Clip that leaves a library out (#56469), or a macOS app whose dependencies include an iOS-only component library (react-native-macos generates the same file), `NSClassFromString` returns nil and the literal throws at launch:

```
 *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[__NSPlaceholderDictionary initWithObjects:forKeys:count:]: attempt to insert nil object from objects[1]'
```

The provider now maps component names to class names and registers the classes the runtime has, the way `RCTModuleProviders.mm` already handles a module provider that cannot be found. A component left out this way falls back to `<UnimplementedView>` if it is rendered, as an unknown component does.

## Changelog:

[IOS] [FIXED] - Skip third-party components whose class is not linked instead of crashing at launch

Pull Request resolved: #59067

Test Plan:
- `generate-artifacts-executor-test.js`: the provider snapshots and the crawl case expect the name-to-class-name map. In Linux (WSL) the file's 52 tests and 24 snapshots pass; its suite-level `fs.rmdirSync(…, {recursive: true})` cleanup fails under Node 26, on `main` too.
- The generated `RCTThirdPartyComponentsProvider.mm` for two component libraries, compiled into a binary that links only one of their classes (clang++, Foundation, macOS):
  - `main`: `NSInvalidArgumentException … attempt to insert nil object from objects[1]`
  - this change: `registered components: LinkedComponent`
- `yarn flow`: no errors; eslint and prettier on the changed files: clean.

Reviewed By: christophpurrer

Differential Revision: D125236957

Pulled By: cipolleschi

fbshipit-source-id: 55126e43352aad7275298978dceaa744a163f966

Author: YevheniiKotyrlo

packages/react-native/scripts/codegen/__tests__/__snapshots__/generate-artifacts-executor-test.js.snap

@@ -328,10 +328,24 @@ exports[`execute test-app "ReactCodegen/RCTThirdPartyComponentsProvider.mm" shou
   static dispatch_once_t nativeComponentsToken;
 
   dispatch_once(&nativeComponentsToken, ^{
-    thirdPartyComponents = @{
-		@\\"TestAppComponent\\": NSClassFromString(@\\"RCTTestAppComponent\\"), // test-app
-		@\\"TestLibraryComponent\\": NSClassFromString(@\\"RCTTestLibraryComponent\\"), // test-library
+    NSDictionary<NSString *, NSString *> * componentMapping = @{
+		@\\"TestAppComponent\\": @\\"RCTTestAppComponent\\", // test-app
+		@\\"TestLibraryComponent\\": @\\"RCTTestLibraryComponent\\", // test-library
     };
+
+    NSMutableDictionary *dict = [[NSMutableDictionary alloc] initWithCapacity:componentMapping.count];
+
+    for (NSString *key in componentMapping) {
+      // Nil for a library this target does not link (an app extension, macOS).
+      Class klass = NSClassFromString(componentMapping[key]);
+      if (!klass) {
+        continue;
+      }
+
+      [dict setObject:klass forKey:key];
+    }
+
+    thirdPartyComponents = dict;
   });
 
   return thirdPartyComponents;
@@ -822,9 +836,23 @@ exports[`execute test-app-legacy "ReactCodegen/RCTThirdPartyComponentsProvider.m
   static dispatch_once_t nativeComponentsToken;
 
   dispatch_once(&nativeComponentsToken, ^{
-    thirdPartyComponents = @{
-		@\\"TestAppDeprecatedComponent\\": NSClassFromString(@\\"RCTTestAppDeprecatedComponentClass\\"), // test-app-legacy
+    NSDictionary<NSString *, NSString *> * componentMapping = @{
+		@\\"TestAppDeprecatedComponent\\": @\\"RCTTestAppDeprecatedComponentClass\\", // test-app-legacy
     };
+
+    NSMutableDictionary *dict = [[NSMutableDictionary alloc] initWithCapacity:componentMapping.count];
+
+    for (NSString *key in componentMapping) {
+      // Nil for a library this target does not link (an app extension, macOS).
+      Class klass = NSClassFromString(componentMapping[key]);
+      if (!klass) {
+        continue;
+      }
+
+      [dict setObject:klass forKey:key];
+    }
+
+    thirdPartyComponents = dict;
   });
 
   return thirdPartyComponents;

packages/react-native/scripts/codegen/__tests__/generate-artifacts-executor-test.js

@@ -297,7 +297,7 @@ describe('generateRCTThirdPartyComponents', () => {
         'utf8',
       );
       expect(generatedFile).toContain(
-        '@"ExampleComponent": NSClassFromString(@"RCTExampleComponent"), // component-library',
+        '@"ExampleComponent": @"RCTExampleComponent", // component-library',
       );
     } finally {
       fs.rmSync(libraryPath, {recursive: true, force: true});

packages/react-native/scripts/codegen/generate-artifacts-executor/generateRCTThirdPartyComponents.js

@@ -139,7 +139,7 @@ function generateRCTThirdPartyComponents(
     .flatMap(library => {
       const components = componentsInLibraries[library];
       return components.map(({componentName, className}) => {
-        return `\t\t@"${componentName}": NSClassFromString(@"${className}"), // ${library}`;
+        return `\t\t@"${componentName}": @"${className}", // ${library}`;
       });
     })
     .join('\n');

packages/react-native/scripts/codegen/templates/RCTThirdPartyComponentsProviderMM.template

@@ -19,9 +19,23 @@
   static dispatch_once_t nativeComponentsToken;
 
   dispatch_once(&nativeComponentsToken, ^{
-    thirdPartyComponents = @{
+    NSDictionary<NSString *, NSString *> * componentMapping = @{
 {thirdPartyComponentsMapping}
     };
+
+    NSMutableDictionary *dict = [[NSMutableDictionary alloc] initWithCapacity:componentMapping.count];
+
+    for (NSString *key in componentMapping) {
+      // Nil for a library this target does not link (an app extension, macOS).
+      Class klass = NSClassFromString(componentMapping[key]);
+      if (!klass) {
+        continue;
+      }
+
+      [dict setObject:klass forKey:key];
+    }
+
+    thirdPartyComponents = dict;
   });
 
   return thirdPartyComponents;