OrionBelt converter drops a field's logical datatype in both directions

Start in ossie_to_obml.py and obml_to_ossie.py, focusing on _convert_field and _convert_column. Read tests/test_ossie_metric_no_silent_loss.py and run the suggested explicit datatype round-trip test. Done means spec datatype values map correctly in both directions and the canonical datatype field is emitted, while existing tests remain green.

Summary

Neither conversion direction carries a field's logical type through the
canonical spec datatype field:

• Ossie->OBML ignores the declared datatype and derives OBML
  abstractType from a column-name heuristic, so a String field can be
  emitted as numeric and vice-versa (wrong type).
• OBML->Ossie never writes the spec datatype field at all - the type is
  stashed only inside an OrionBelt custom_extensions blob (missing type).

A round-trip appears lossless only because the converter is self-consistent
with its own non-spec key; any other spec-compliant tool sees wrong or absent
types.

Root cause

Ossie->OBML (ossie_to_obml.py):

ossie_type = field.get("data_type", "")               # (1) wrong key
if ossie_type and ossie_type in OSSIE_TO_OBML_TYPE:    # (2) wrong-cased map
    abstract_type = OSSIE_TO_OBML_TYPE[ossie_type]
else:
    abstract_type = self._infer_obml_type(field)

1. Wrong key. The spec field is datatype, no underscore, so the branch is dead
2. Wrong-cased map. The spec enum is capitalized, but
   OSSIE_TO_OBML_TYPE is keyed lowercase with different terms.
   Even after fixing the key, case-normalizing is not enough:
   DateTime/DateTimeTz/Decimal/Float have no key - so case-normalizing is not enough

OBML->Ossie: canonical field never written (obml_to_ossie.py).
The type goes only into a vendor extension, under the non-spec key
data_type with a lowercase value; field["datatype"] is not set.

Illustrative Repro

# Ossie -> OBML: declared datatype ignored
for f, want in [({"name": "total_amount", "datatype": "String"},  "string"),
                ({"name": "customer_id",  "datatype": "Integer"}, "int")]:
    _, col = OssietoOBML(ossie={})._convert_field(dict(f))
    got = col["abstractType"]
    print(f"fwd {f['datatype']:8} -> {got:8} (want {want}) [{'OK' if got==want else 'BUG'}]")

# OBML -> Ossie: canonical datatype never emitted
field = OBMLtoOssie(obml={})._convert_column("customer_id", {"code": "customer_id",
                    "abstractType": "int"}, "Orders", {})
print("rev canonical 'datatype' present?", "datatype" in field, "[BUG]" if "datatype" not in field else "")

Why this survived

The existing fixtures (test_ossie_converter_vendors, test_ossie_metric_no_silent_loss)
use the same wrong key data_type, so the suite stays green with the bug present.

Drop-in test (currently fails). Suggested home:
tests/test_ossie_metric_no_silent_loss.py, which already guards this "no
silent loss on field conversion" path.

def test_explicit_datatype_roundtrips():
    _, col = OssietoOBML(ossie={})._convert_field({"name": "total_amount", "datatype": "String"})
    assert col["abstractType"] == "string"          # fwd: currently 'float'
    field = OBMLtoOssie(obml={})._convert_column("total_amount",
                {"code": "total_amount", "abstractType": "string"}, "Orders", {})
    assert field.get("datatype") == "String"        # rev: currently missing

Suggested fix

• Ossie->OBML: read field.get("datatype") (fix the comment); rebuild
  OSSIE_TO_OBML_TYPE on the spec enum
• OBML->Ossie: set the canonical field["datatype"] (capitalized enum value)
  in addition to any extension bookkeeping.