feat(GHC): add code review instructions (#63586)

Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>

.github/instructions/testing.instructions.md

@@ -0,0 +1,103 @@
+---
+applyTo: "**/*.{ts,tsx,js,jsx}"
+---
+
+# Test Coverage Review Instructions
+
+Review code changes for adequate test coverage using freeCodeCamp's testing conventions.
+
+## Testing Framework Reference
+
+| Location | Framework | File Pattern | Notes |
+|----------|-----------|--------------|-------|
+| `api/` | Vitest | `*.test.ts` | Co-located with source |
+| `client/` | Vitest | `*.test.{ts,tsx}` | Co-located with source |
+| `e2e/` | Playwright | `*.spec.ts` | Dedicated directory |
+
+## When to Flag Missing Tests
+
+Comment on missing tests if ALL of these are true:
+
+- PR modifies functional code (bug fix or new feature)
+- No corresponding test additions or modifications exist
+- Changes are not test-only or config-only
+
+### Examples Requiring Tests
+
+- New utility functions in `api/src/utils/` or `client/src/utils/`
+- New API route handlers in `api/src/routes/`
+- New React components with logic in `client/src/components/`
+- Bug fixes that change application behavior
+- New validation logic or data transformations
+
+## When NOT to Flag
+
+Do not comment on test coverage if:
+
+- Changes are documentation, configuration, or non-functional
+- PR already includes appropriate test coverage
+- Changes are to test files themselves
+- Changes are trivial:
+  - Import reorganization
+  - Formatting changes
+  - Type-only changes (interfaces, type definitions)
+  - Comment updates
+- Changes are in areas without existing test patterns:
+  - Curriculum markdown files
+  - Configuration files
+  - Build scripts
+
+## Detecting Outdated Tests
+
+Flag if existing tests may be outdated:
+
+- Test file exists for modified source but doesn't cover the changed functionality
+- Test assertions reference behavior that is being changed
+- Mock data doesn't reflect new data structures or API responses
+- Test descriptions no longer match actual test behavior
+
+### Example Comment for Outdated Tests
+
+```
+The existing tests in `src/utils/validate.test.ts` may need updates to cover the new validation rules added in this PR.
+```
+
+## Comment Format
+
+### Missing Tests
+
+Keep feedback specific and actionable:
+
+```
+Consider adding tests for the changes in `src/utils/validate.ts`.
+Suggested coverage:
+- Valid input returns expected output
+- Invalid input throws appropriate error
+- Edge case: empty string handling
+```
+
+### Sufficient Coverage
+
+```
+LGTM
+```
+
+### Outdated Tests
+
+```
+The tests in `[test file]` may need updates to reflect the changes to `[source file]`.
+Specifically:
+- [what behavior changed]
+- [what test assertions may be affected]
+```
+
+## Test Quality Indicators
+
+When tests are present, briefly verify:
+
+- Tests cover the happy path
+- Tests cover at least one error/edge case
+- Test descriptions are meaningful (not just "test 1", "test 2")
+- Mocks are appropriate (not mocking the thing being tested)
+
+Do not block PRs for test style preferences if coverage is adequate.