UI: Ember copilot instruction updates (#11491) (#11501)

* [VAULT-33083] UI: Updating ember copilot instructions to avoid using mixins and use utility functions instead

* add github instruction to avoid unnecessary exports

* add copilot test instructions so that test files should only test repository-specific logic. test coverage should not test framework or external library functionality.

Co-authored-by: Shannon Roberts (Beagin) <beagins@users.noreply.github.com>

.github/instructions/generic/ember_js.instructions.md

@@ -20,6 +20,7 @@ This document provides JavaScript and TypeScript coding standards for HashiCorp
 - Remove all unused imports, variables, and functions before committing
 - Place comments directly above the code they describe, not inline or below
 - Update comments when code changes to maintain accuracy
+- Only export functions, classes, or variables that are used by other files. If a function is only used within the same file, keep it private (no export). This reduces API surface area and improves maintainability.
 
 ## TypeScript Guidelines
 - **File Naming**: All new files should use `.ts` extension instead of `.js`
@@ -36,6 +37,28 @@ This document provides JavaScript and TypeScript coding standards for HashiCorp
 - Co-locate component templates (`.hbs`) with their TypeScript files (`.ts` preferred over `.js`)
 - Prioritize reusability and maintainability when creating components - avoid overly complex or one-off implementations
 
+## Deprecated Patterns to Avoid
+- **Don't use Mixins** 
+- Ember mixins (`Mixin.create()`) are deprecated and being phased out
+- Convert existing mixin functionality to utility modules or service injection instead
+- Use utility functions or services for shared logic between classes
+- **Code Review**: Flag any imports from `@ember/object/mixin` or `*.extend(SomeMixin)` patterns
+
+```javascript
+// DEPRECATED: Don't use mixins
+import Mixin from '@ember/object/mixin';
+import SomeMixin from 'vault/mixins/some-mixin';
+export default Route.extend(SomeMixin, { /* ... */ });
+
+// Instead, use utility functions
+import { utilityFunction } from 'vault/utils/utility-helpers';
+export default class MyRoute extends Route {
+  someMethod() {
+    return utilityFunction(this);
+  }
+}
+```
+
 ## Asynchronous Programming
 - Use `async`/`await` with proper error handling in `try`/`catch` blocks
 - Only use `@task` from ember-concurrency when you need specific features like cancellation or `task.isRunning` state management

.github/instructions/generic/ember_tests.instructions.md

@@ -25,6 +25,47 @@ This document provides testing standards and best practices for HashiCorp Ember.
 
 # Testing Standards
 
+## Test Focus and Purpose
+- **Test repository-specific business logic**, not framework internals or external library functionality
+- Focus on testing the behavior and logic that your application implements
+- Avoid testing that framework methods exist or that imported functions are functions
+- Test how your code interacts with frameworks, not the frameworks themselves
+
+**What TO test:**
+- Business logic and application-specific behavior
+- Component interactions and state changes
+- Data transformations and computations
+- User workflows and edge cases
+- Error handling and validation logic
+
+**What NOT to test:**
+- That Ember route methods like `modelFor()` or `paramsFor()` exist
+- That imported utility functions are of type 'function'
+- Framework-provided functionality (Ember, QUnit, etc.)
+- Third-party library internals
+- Basic JavaScript language features
+
+Example of unnecessary framework testing:
+```javascript
+// Don't do this - testing framework internals
+test('route has standard methods', function (assert) {
+  assert.ok(this.route.modelFor, 'Route has modelFor method');
+  assert.strictEqual(typeof myUtilFunction, 'function', 'function is a function');
+});
+```
+
+Example of proper business logic testing:
+```javascript  
+// Do this - test your application logic
+test('getBackendEffectiveType returns correct type for external plugins', function (assert) {
+  this.route.modelFor = this.stub().returns({ engineType: 'vault-plugin-secrets-keymgmt' });
+  
+  const effectiveType = getBackendEffectiveType(this.route);
+  
+  assert.strictEqual(effectiveType, 'keymgmt', 'External keymgmt plugin returns effective type');
+});
+```
+
 ## Test Quality
 - Use `assert.true()` or `assert.false()` instead of `assert.ok()` for boolean checks
 - Provide descriptive assertion messages that explain what is being verified