Surface deep research probe errors (#1086)

Co-authored-by: ghreprimand <203024559+ghreprimand@users.noreply.github.com>

tests/test_research_probe_errors.py

@@ -0,0 +1,61 @@
+"""Regression tests for Deep Research model probe error messages.
+
+Deep Research probes the selected model before starting a long run. When the
+upstream returned a concrete model/API error, the probe used to collapse it into
+"Cannot reach model", hiding the real issue from the UI.
+"""
+import pytest
+from fastapi import HTTPException
+
+from src.research_handler import ResearchHandler, _format_probe_failure
+
+
+def test_probe_failure_preserves_upstream_model_errors():
+    exc = HTTPException(
+        status_code=400,
+        detail="OpenAI returned HTTP 400: Unsupported parameter: temperature",
+    )
+
+    msg = _format_probe_failure("o3-mini", exc)
+
+    assert msg == (
+        "Model 'o3-mini' probe failed: "
+        "OpenAI returned HTTP 400: Unsupported parameter: temperature"
+    )
+
+
+def test_probe_failure_keeps_api_key_guidance():
+    exc = HTTPException(status_code=401, detail="OpenAI authentication failed")
+
+    assert _format_probe_failure("gpt-4o", exc) == (
+        "Model 'gpt-4o' requires an API key. Check your endpoint configuration."
+    )
+
+
+def test_probe_failure_keeps_reachability_guidance_for_plain_errors():
+    msg = _format_probe_failure("local-model", RuntimeError("connection refused"))
+
+    assert msg == "Cannot reach model 'local-model' — connection refused"
+
+
+@pytest.mark.asyncio
+async def test_probe_endpoint_surfaces_http_exception_detail(monkeypatch):
+    async def _raise(*args, **kwargs):
+        raise HTTPException(
+            status_code=400,
+            detail="OpenAI returned HTTP 400: max_tokens is not supported",
+        )
+
+    monkeypatch.setattr("src.llm_core.llm_call_async", _raise)
+
+    with pytest.raises(RuntimeError) as excinfo:
+        await ResearchHandler._probe_endpoint(
+            "https://api.openai.com/v1/chat/completions",
+            "o3-mini",
+            {"Authorization": "Bearer test"},
+        )
+
+    msg = str(excinfo.value)
+    assert "Model 'o3-mini' probe failed" in msg
+    assert "max_tokens is not supported" in msg
+    assert "Cannot reach model" not in msg