fix(coding-agent): fixed crashes when system prompts were provided as strings

- Normalized agent `setSystemPrompt` to wrap string inputs into one-item arrays.
- Updated session creation to accept string `systemPrompt` values and normalize callback or direct results to string arrays.
- Adjusted extension result handling and test fixtures to accept string `systemPrompt` and missing `assistant_message` fields without crashing.

python/robomp/tests/test_worker.py

@@ -70,6 +70,7 @@ class _FakeRpcClient:
             messages: list = []
             events: list = []
             assistant_text: str = "ok"
+            assistant_message: dict | None = None
 
         return _Turn()