Fix GEN_KW update flag matching parameter name

The update flag was set using `"CONST" not in params`,
which searches the entire list including the parameter
name. A parameter named "CONST" with a non-CONST
distribution would incorrectly get update=False.

Narrow the check to only inspect the distribution type
at params[1].

Author: dafeda

src/ert/config/gen_kw_config.py

@@ -194,6 +194,7 @@ def from_config_list(cls, config_list: GenKwConfigList) -> list[Self]:
                 "to exclude this from updates, set UPDATE:FALSE.\n",
                 gen_kw_key,
             )
+        # "CONST" not in params and parsed.option.update
         try:
             return [
                 cls(
@@ -203,7 +204,7 @@ def from_config_list(cls, config_list: GenKwConfigList) -> list[Self]:
                         params[0], params[1], params[2:]
                     ),
                     forward_init=False,
-                    update="CONST" not in params and parsed.options.update,
+                    update=params[1] != "CONST" and parsed.options.update,
                 )
                 for params in distributions_spec
             ]

tests/ert/unit_tests/config/test_gen_kw_config.py

@@ -861,6 +861,25 @@ def test_that_const_keyword_sets_update_to_false(tmpdir):
         assert gen_kw_config.update is False
 
 
+def test_that_parameter_named_const_with_non_const_distribution_is_updatable(tmp_path):
+    """Regression: update flag should only check the distribution type,
+    not match "CONST" anywhere in the parameter definition.
+    """
+    parameter_file = tmp_path / "parameter.txt"
+    parameter_file.write_text("CONST NORMAL 0 1", encoding="utf-8")
+
+    configs = GenKwConfig.from_config_list(
+        [
+            "MY_KW",
+            (str(parameter_file), parameter_file.read_text(encoding="utf-8")),
+            {},
+        ]
+    )
+    assert len(configs) == 1
+    assert configs[0].name == "CONST"
+    assert configs[0].update is True
+
+
 def test_that_unexpected_positional_arg_count_raises_validation_error(tmp_path):
     parameter_file = tmp_path / "parameter.txt"
     parameter_file.write_text("KEY NORMAL 0 1", encoding="utf-8")