Add support for test generation when manual test is provided

[Jack-Khuu]

tldr: Add support to still generate tests when a custom one is provided to TKA.generate_kernel.

Currently, when a custom test is provided, the pipeline does not attempt to generate an LLM test for the problem.
This is fine, but the generated testing is very useful to have even if there is a custom test (e.g. if the custom test is expensive to run or incomplete).

This PR adds support for generating this test via a generate_default_test flag.

• To support this, the test_code signature throughout the repo needed to be updated from str to list[str]. Most of the files are changed, just change the type for test_code from str => list[str].

The only interesting changes are:

• triton_kernel_agent/agent.py and triton_kernel_agent/worker.py
  When multiple tests are provided, code extraction will prefer the code block with kernel_function in it.
  Extraction is unchanged for single tests

• triton_kernel_agent/worker_util.py
  Helper for formatting the additional tests as context

f"\n\n# === ADDITIONAL VALIDATION TEST {i + 1} "
            f"(for reference only — do NOT copy into your kernel) ===\n"

---

Note: this does not enable secondary tests for the Fuser path. This would require a separate effort as testing occurs only during composition which is after the sub-kernels have already been built

This use case will be used for integrating auxiliary testing (internally)

---

Tested internally via

result = agent.generate_kernel(
        problem_description=problem_description,
        test_code=submission_test_code,
        generate_default_test=True,
    )

result = agent.generate_kernel(
        problem_description=problem_description,
        test_code=submission_test_code,
        generate_default_test=False,
    )

[meta-codesync]

@Jack-Khuu has imported this pull request. If you are a Meta employee, you can view this in D95858679.

[kaiming-cheng]

Overall looks good! My comments is mainly on the design of using a list to store different test cases. Let me know what you think!