Have ModelConfig output more noticable warning when malformatted runpath

This commit makes ModelConfig emit a ConfigWarning if the input runpath does not contain `<ITER>` or `<IENS>`. This was previously only a warning in the logs, but it should be more noticable.

src/ert/config/model_config.py

@@ -8,7 +8,13 @@
 from pydantic import field_validator
 from pydantic.dataclasses import dataclass
 
-from .parsing import ConfigDict, ConfigKeys, ConfigValidationError, HistorySource
+from .parsing import (
+    ConfigDict,
+    ConfigKeys,
+    ConfigValidationError,
+    ConfigWarning,
+    HistorySource,
+)
 
 logger = logging.getLogger(__name__)
 
@@ -70,11 +76,13 @@ def validate_runpath(cls, runpath_format_string: str) -> str:
             )
         result = _replace_runpath_format(runpath_format_string)
         if not any(x in result for x in ["<ITER>", "<IENS>"]):
-            logger.warning(
+            msg = (
                 "RUNPATH keyword contains no value placeholders: "
                 f"`{runpath_format_string}`. Valid example: "
                 f"`{DEFAULT_RUNPATH}` "
             )
+            ConfigWarning.warn(msg)
+            logger.warning(msg)
         return result
 
     @field_validator("jobname_format_string", mode="before")

tests/ert/unit_tests/config/test_ert_config.py

@@ -1852,3 +1852,20 @@ def test_no_warning_when_summary_key_and_simulation_job_present(job_name, key):
     with warnings.catch_warnings():
         warnings.simplefilter("error", category=ConfigWarning)
         ErtConfig.from_file("config_file.ert")
+
+
+def test_warning_is_emitted_when_malformatted_runpath(tmp_path):
+    config_path = tmp_path / "config_file.ert"
+    with open(config_path, "w", encoding="utf-8") as fout:
+        # Write a minimal config file
+        fout.write(
+            "RUNPATH <STORAGE>/runpath/constant-realization-num/contant-iter-num\n"
+        )
+        fout.write("NUM_REALIZATIONS 1\n")
+    with warnings.catch_warnings(record=True) as all_warnings:
+        ErtConfig.from_file(config_path)
+    assert any(
+        ("RUNPATH keyword contains no value placeholders" in str(w.message))
+        for w in all_warnings
+        if isinstance(w.message, ConfigWarning)
+    )