fix(rlsapi): use customization.system_prompt in /v1/infer

- Check configuration.customization.system_prompt before falling back
  to constants.DEFAULT_SYSTEM_PROMPT in _build_instructions()
- Add unit tests for custom prompt, None prompt, and None customization

Signed-off-by: Major Hayden <major@redhat.com>

Author: major

src/app/endpoints/rlsapi_v1.py

@@ -92,7 +92,13 @@ def _build_instructions(systeminfo: RlsapiV1SystemInfo) -> str:
     Returns:
         Instructions string for the LLM, with system context if available.
     """
-    base_prompt = constants.DEFAULT_SYSTEM_PROMPT
+    if (
+        configuration.customization is not None
+        and configuration.customization.system_prompt is not None
+    ):
+        base_prompt = configuration.customization.system_prompt
+    else:
+        base_prompt = constants.DEFAULT_SYSTEM_PROMPT
 
     context_parts = []
     if systeminfo.os:

tests/unit/app/endpoints/test_rlsapi_v1.py

@@ -167,6 +167,55 @@ def test_build_instructions(
         assert not_expected not in result
 
 
+# --- Test _build_instructions with customization.system_prompt ---
+
+
+@pytest.mark.parametrize(
+    ("custom_prompt", "expected_prompt"),
+    [
+        pytest.param(
+            "You are a RHEL expert.",
+            "You are a RHEL expert.",
+            id="customization_system_prompt_set",
+        ),
+        pytest.param(
+            None,
+            constants.DEFAULT_SYSTEM_PROMPT,
+            id="customization_system_prompt_none",
+        ),
+    ],
+)
+def test_build_instructions_with_customization(
+    mocker: MockerFixture,
+    custom_prompt: str | None,
+    expected_prompt: str,
+) -> None:
+    """Test _build_instructions uses customization.system_prompt when set."""
+    mock_customization = mocker.Mock()
+    mock_customization.system_prompt = custom_prompt
+    mock_config = mocker.Mock()
+    mock_config.customization = mock_customization
+    mocker.patch("app.endpoints.rlsapi_v1.configuration", mock_config)
+
+    systeminfo = RlsapiV1SystemInfo(os="RHEL", version="9.3", arch="x86_64")
+    result = _build_instructions(systeminfo)
+
+    assert expected_prompt in result
+    assert "OS: RHEL" in result
+
+
+def test_build_instructions_no_customization(mocker: MockerFixture) -> None:
+    """Test _build_instructions falls back when customization is None."""
+    mock_config = mocker.Mock()
+    mock_config.customization = None
+    mocker.patch("app.endpoints.rlsapi_v1.configuration", mock_config)
+
+    systeminfo = RlsapiV1SystemInfo()
+    result = _build_instructions(systeminfo)
+
+    assert result == constants.DEFAULT_SYSTEM_PROMPT
+
+
 # --- Test _get_default_model_id ---