Add unittests for API endpoint

[gpsandhu23]

Adds unit tests for the API endpoints in api_test.py to ensure correctness and reliability of the API in various scenarios.

• Setup and Dependencies: Initializes TestClient with the FastAPI app and imports necessary modules including unittest, fastapi.testclient, and AgentManager.
• Successful API Call Test: Implements test_chat_endpoint_success to verify that the API returns a 200 status code and a string response when provided with valid input.
• Invalid Input Test: Adds test_chat_endpoint_invalid_input to check the API's response to missing input, expecting a 422 status code.
• Error Handling Test: Creates test_chat_endpoint_error_handling to simulate an internal server error using a mock object, verifying that the API correctly returns a 500 status code and an appropriate error message.

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[sweep-ai[bot]]

Sweep: PR Review

TARS/tests/surfaces/API/api_test.py

The changes add a new test suite for the /chat endpoint of the FastAPI application, including tests for successful responses, invalid input handling, and error handling.

Sweep Found These Issues

• The unittest.mock.patch.object is used without importing unittest.mock, which will cause a NameError.
https://github.com/gpsandhu23/TARS/blob/e718df7f075a1bbd983270ebc8a41d711b23b9e2/TARS%2Ftests%2Fsurfaces%2FAPI%2Fapi_test.py#L20-L24 [View Diff](https://github.com/gpsandhu23/TARS/pull/50/files#diff-770678a4c4ef1f405e6ec1601d7e1e492a5dca9f4a2b17eb976eec30ec46543eR20-R24)

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