fix(provider): accept plain text OpenAI-compatible responses

Handle string and dict-shaped responses from OpenAI-compatible backends so non-standard providers no longer crash on missing choices fields. Add regression tests to keep SDK, dict, and plain-text parsing paths aligned.

tests/providers/test_custom_provider.py

@@ -15,3 +15,41 @@ def test_custom_provider_parse_handles_empty_choices() -> None:
 
     assert result.finish_reason == "error"
     assert "empty choices" in result.content
+
+
+def test_custom_provider_parse_accepts_plain_string_response() -> None:
+    with patch("nanobot.providers.openai_compat_provider.AsyncOpenAI"):
+        provider = OpenAICompatProvider()
+
+    result = provider._parse("hello from backend")
+
+    assert result.finish_reason == "stop"
+    assert result.content == "hello from backend"
+
+
+def test_custom_provider_parse_accepts_dict_response() -> None:
+    with patch("nanobot.providers.openai_compat_provider.AsyncOpenAI"):
+        provider = OpenAICompatProvider()
+
+    result = provider._parse({
+        "choices": [{
+            "message": {"content": "hello from dict"},
+            "finish_reason": "stop",
+        }],
+        "usage": {
+            "prompt_tokens": 1,
+            "completion_tokens": 2,
+            "total_tokens": 3,
+        },
+    })
+
+    assert result.finish_reason == "stop"
+    assert result.content == "hello from dict"
+    assert result.usage["total_tokens"] == 3
+
+
+def test_custom_provider_parse_chunks_accepts_plain_text_chunks() -> None:
+    result = OpenAICompatProvider._parse_chunks(["hello ", "world"])
+
+    assert result.finish_reason == "stop"
+    assert result.content == "hello world"