feat(documentation): Add documentation for RetakeScheduler tests

doc/tests/RetakeScheduler.md

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+# RetakeScheduler Node Unit Tests
+
+Unit tests for `RetakeScheduler` are implemented in `src/g2_2025_grade_calculator_pkg/test/RetakeScheduler.test.cpp`
+The unit tests are written using Google Test and ROS 2 test utilities. They utilize clients to interact
+with the RetakeScheduler node's services and actions, verifying correct behavior and expected outcomes.
+
+## Test Cases
+
+### 1. ConstructorTest
+
+- **Purpose:**
+  Verifies that the `RetakeScheduler` can be constructed without throwing exceptions.
+- **Checks:**
+  - The node is created successfully and is not `nullptr`.
+
+
+### 2. ActionClientCreationTest
+
+- **Purpose:**
+  Ensures that the `RetakeScheduler` node creates the expected action client services for the retake action.
+- **Checks:**
+  - At least one of the retake action client services (`/retake_action/_action/cancel_goal`, `/retake_action/_action/get_result`, `/retake_action/_action/send_goal`) is present in the node's service graph.
+
+
+### 3. ParameterTest
+
+- **Purpose:**
+  Confirms that the node's parameters are set to their expected default values.
+- **Checks:**
+  - The `retake_check_interval_sec` parameter defaults to `120`.
+
+
+### 4. ActionClientAllServicesPresentTest
+
+- **Purpose:**
+  Verifies that all required action client services for the retake action are available.
+- **Checks:**
+  - The following services are present:
+    - `/retake_action/_action/cancel_goal`
+    - `/retake_action/_action/get_result`
+    - `/retake_action/_action/send_goal`
+
+
+### 5. ConstructorWithNullDatabaseManager
+
+- **Purpose:**
+  Tests the node's behavior when constructed with a `nullptr` database manager.
+- **Checks:**
+  - The node can be constructed without throwing, even if the database manager is `nullptr`.
+