chore: import upstream snapshot with attribution

skills/angular-developer/references/injection-context.md

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+# Injection Context
+
+The `inject()` function can only be used when code is executing within an **injection context**.
+
+## Where is an Injection Context Available?
+
+An injection context is automatically available in:
+
+1. **Field initializers** of classes instantiated by DI (`@Injectable`, `@Component`, `@Directive`, `@Pipe`).
+2. **Constructors** of classes instantiated by DI.
+3. **Factory functions** specified in `useFactory` or `InjectionToken` configurations.
+4. **Functional APIs** executed by Angular (e.g., functional route guards, resolvers, interceptors).
+
+```ts
+@Component({...})
+export class Example {
+  // Valid: Field initializer
+  private router = inject(Router);
+
+  constructor() {
+    // Valid: Constructor
+    const http = inject(HttpClient);
+  }
+
+  onClick() {
+    // Invalid: Not an injection context
+    // const auth = inject(AuthService);
+  }
+}
+```
+
+## `runInInjectionContext`
+
+If you need to run a function within an injection context (often needed for dynamic component creation or testing), use `runInInjectionContext`. This requires access to an existing injector (like `EnvironmentInjector` or `Injector`).
+
+```ts
+import {Injectable, inject, EnvironmentInjector, runInInjectionContext} from '@angular/core';
+
+@Injectable({providedIn: 'root'})
+export class MyService {
+  private injector = inject(EnvironmentInjector);
+
+  doSomethingDynamic() {
+    runInInjectionContext(this.injector, () => {
+      // Now valid to use inject() here
+      const router = inject(Router);
+    });
+  }
+}
+```
+
+## `assertInInjectionContext`
+
+Use `assertInInjectionContext` in utility functions to guarantee they are called from a valid context. It throws a clear error if not.
+
+```ts
+import {assertInInjectionContext, inject, ElementRef} from '@angular/core';
+
+export function injectNativeElement<T extends Element>(): T {
+  assertInInjectionContext(injectNativeElement);
+  return inject(ElementRef).nativeElement;
+}
+```