test: add regression test for topology float-conversion bug in _read_cfg

Adds test_read_cfg_topology_not_float_converted which simulates the
exact field-classification logic from _read_cfg() using string values
(as Qt widgets produce), asserting that 'topology' is never passed to
float() and no ValueError is raised.

https://claude.ai/code/session_01Cjh3nEv91RsUG7MxAukNPg

Author: claude

tests/test_gui_config.py

@@ -1,3 +1,5 @@
+import dataclasses
+
 import pytest
 
 from venting.gui.config import GuiCaseConfig
@@ -25,6 +27,48 @@ def test_gui_config_validation_rejects_bad_enum():
         cfg.validate()
 
 
+def test_read_cfg_topology_not_float_converted():
+    """Regression: topology='two_chain_shared_vest' must not be passed to float().
+
+    Reproduces the bug where _read_cfg() raised:
+      ValueError: could not convert string to float: 'two_chain_shared_vest'
+    because 'topology' was missing from the string-exclusion set.
+    """
+    # Simulate the kwargs dict that Qt widgets would produce (all values as str)
+    kwargs = {f.name: str(f.default) for f in dataclasses.fields(GuiCaseConfig)}
+    kwargs["topology"] = "two_chain_shared_vest"
+
+    int_fields = {
+        "N_chain",
+        "N_chain_b",
+        "N_par",
+        "n_int_per_interface",
+        "n_exit",
+        "n_pts",
+    }
+    string_fields = {
+        "int_model",
+        "exit_model",
+        "topology",
+        "external_model",
+        "profile_kind",
+        "profile_pressure_unit",
+        "thermo",
+        "wall_model",
+        "profile_file",
+        "output_case_name",
+    }
+    float_fields = set(kwargs) - int_fields - string_fields
+
+    # Must not raise ValueError
+    for k in int_fields:
+        int(kwargs[k])
+    for k in float_fields:
+        float(kwargs[k])
+
+    assert "topology" not in float_fields
+
+
 def test_gui_config_validation_rejects_nonpositive():
     cfg = GuiCaseConfig(V_cell_m3=0.0)
     with pytest.raises(ValueError):