FIX: Convert fluid contact to lowercase

This is for backward compatibility with existing models. It is probably
not a pattern we want to carry forward. This also emits a warning for
this particular case that uppercase characters may be disallowed in the
future.

src/fmu/dataio/datastructure/meta/content.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import warnings
 from typing import Any, Dict, List, Literal, Optional, Union
 
 from pydantic import (
@@ -9,6 +10,7 @@
     GetJsonSchemaHandler,
     NaiveDatetime,
     RootModel,
+    field_validator,
     model_validator,
 )
 from pydantic_core import CoreSchema
@@ -78,6 +80,18 @@ class FluidContact(BaseModel):
     )
     truncated: bool = Field(default=False)
 
+    @field_validator("contact", mode="before")
+    def contact_to_lowercase(cls, v: str) -> str:
+        if any(c.isupper() for c in v):
+            warnings.warn(
+                f"You've defined the fluid contact as '{v}' which contains uppercase "
+                "characters. In a future version we may require that fluid contacts "
+                "should be all lowercase. To ensure future compatibility you should "
+                f"change this value to '{v.lower()}'.",
+                UserWarning,
+            )
+        return v.lower()
+
 
 class FieldOutline(BaseModel):
     """

src/fmu/dataio/types.py

@@ -50,7 +50,6 @@ class PolygonsProxy(Polygons): ...
         "Collection of 'inferrable' objects with metadata deduction capabilities",
     ]
 
-
 Parameters: TypeAlias = Annotated[
     MutableMapping[str, Union[str, float, int, None, "Parameters"]],
     "Nested or flat configurations for dynamically structured parameters.",

tests/test_units/test_contents.py

@@ -1,6 +1,8 @@
 """Explicitly test all allowed contents."""
 
+import pytest
 from fmu.dataio.dataio import ExportData
+from pydantic import ValidationError
 
 # generic testing of functionality related to content is done elsewhere,
 # mainly in test_dataio.py.
@@ -66,6 +68,28 @@ def test_content_fluid_contact(regsurf, globalconfig2):
     assert meta["data"]["content"] == "fluid_contact"
 
 
+def test_content_fluid_contact_case_insensitive(regsurf, globalconfig2):
+    """Test export of the fluid_contact content."""
+    with pytest.warns(UserWarning, match=r"contains uppercase.+value to 'owc'"):
+        meta = ExportData(
+            config=globalconfig2,
+            name="MyName",
+            content={"fluid_contact": {"contact": "OWC"}},
+        ).generate_metadata(regsurf)
+
+    assert meta["data"]["fluid_contact"]["contact"] == "owc"
+
+
+def test_content_fluid_contact_raises_on_invalid_contact(regsurf, globalconfig2):
+    """Test export of the fluid_contact content."""
+    with pytest.raises(ValidationError, match="fluid_contact"):
+        ExportData(
+            config=globalconfig2,
+            name="MyName",
+            content={"fluid_contact": {"contact": "OEC"}},
+        ).generate_metadata(regsurf)
+
+
 def test_content_kh_product(regsurf, globalconfig2):
     """Test export of the khproduct content."""
     meta = ExportData(