fix(python): expose function source digests (#4729)

Function descriptions include `source_digest`, but both the Rust
response model and Python `FunctionVersion` discarded it. Clients
therefore could not compare a catalog entry with
`udf_def.registration_request.artifact.digest`.

Preserve the digest through Rust serialization and expose it on Python
`FunctionVersion`, with a documented Rust accessor. Missing or null
digests decode as `None`, keeping older responses compatible. Regression
tests cover serialization, immutability, registration jobs, get, and
synchronous/asynchronous list responses while keeping the image digest
independent.

Validation:
- Confirmed that the original Python model silently discarded a
server-provided source digest.
- Rebuilt the Python extension in the uv-managed environment; both
focused Function test files passed (106 tests).
- Focused Rust Function model tests passed (10 tests).
- Full workspace `cargo check --quiet --features remote --tests
--examples` and `cargo clippy --quiet --features remote --tests
--examples` passed.
- Rust formatting and Ruff checks/formatting for changed Python files
passed.
- Repository-wide Ruff reports 138 existing violations in unchanged
docs/CI files.

Fixes #4723

<!-- lance-gatekeeper-fix:v1 agent=c145f25136f7b908217e04bde8a2548b
generation=1 -->

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>

python/python/lancedb/functions.py

@@ -366,6 +366,12 @@ class FunctionVersion(_RemoteValue):
     [SecretReference][lancedb.functions.SecretReference] carries a Secret's;
     they are what [get_function][lancedb.db.DBConnection.get_function] takes.
     An empty ``namespace_path`` is the root namespace.
+
+    ``source_digest`` identifies the registered source artifact, independently
+    of the built ``image``. Compare it with
+    ``udf_def.registration_request.artifact.digest`` to check whether a local
+    [UdfDefinition][lancedb.functions.UdfDefinition] has the same source.
+    It is ``None`` when the server does not provide a source digest.
     """
 
     name: str
@@ -374,6 +380,7 @@ class FunctionVersion(_RemoteValue):
     location: str
     version: _ObjectVersion
     image: FunctionImage
+    source_digest: Optional[str] = None
     signature: FunctionSignature
     secret_bindings: tuple[SecretBinding, ...] = ()
     created_at_millis: int

python/python/tests/test_first_class_function_slice1.py

@@ -146,6 +146,27 @@ def test_function_version_identity_is_immutable_and_exact():
             FunctionVersion(**{**value, "version": invalid})
 
 
+@pytest.mark.parametrize("source_digest", [None, "sha256:source-artifact"])
+def test_function_version_preserves_source_digest(source_digest):
+    value = job_result("remote_function_job.json")
+    legacy = FunctionVersion.from_json(json.dumps(value))
+    assert legacy.source_digest is None
+    assert "source_digest" not in json.loads(legacy.to_canonical_json())
+
+    value["source_digest"] = source_digest
+    version = FunctionVersion.from_json(json.dumps(value))
+    assert version.source_digest == source_digest
+    assert version.image == legacy.image
+    encoded = version.to_canonical_json()
+    if source_digest is None:
+        assert "source_digest" not in json.loads(encoded)
+    else:
+        assert json.loads(encoded)["source_digest"] == source_digest
+    assert FunctionVersion.from_json(encoded) == version
+    with pytest.raises((TypeError, ValueError)):
+        version.source_digest = "sha256:changed"
+
+
 def test_function_version_binds_named_columns_as_one_immutable_application():
     version = FunctionVersion.from_json(
         json.dumps(job_result("remote_function_job.json"))

python/python/tests/test_first_class_function_slice2.py

@@ -1651,6 +1651,7 @@ def _mock_remote_function_catalog():
                         ).read_text()
                     )["image"],
                     "signature": body["signature"],
+                    "source_digest": body["artifact"]["digest"],
                     "secret_bindings": body.get("secret_bindings", []),
                     "created_at_millis": 1787270400000,  # 2026-08-21T00:00:00Z
                 }
@@ -1760,6 +1761,9 @@ def test_remote_registration_job_and_exact_version_reopen_round_trip():
     assert created == reopened
     assert reopened.name == "normalize_score"
     assert reopened.version == FUNCTION_VERSION
+    assert (
+        reopened.source_digest == normalize_score.registration_request.artifact.digest
+    )
     create_request = state["requests"][0][1]
     expected_request = json.loads(
         normalize_score.registration_request.to_canonical_json()
@@ -1866,6 +1870,7 @@ def test_blocking_remote_registration_returns_function_version():
 
     assert created.name == "normalize_score"
     assert created.version == FUNCTION_VERSION
+    assert created.source_digest == normalize_score.registration_request.artifact.digest
     assert [path for path, _ in state["requests"]] == [
         "/v1/function/normalize_score/create",
         "/v1/jobs/describe",
@@ -1885,6 +1890,9 @@ def test_remote_list_functions_paginates_and_returns_typed_versions():
         functions = list(db.list_functions())
 
     assert functions == [created]
+    assert functions[0].source_digest == (
+        normalize_score.registration_request.artifact.digest
+    )
     assert state["requests"] == [
         ("/v1/namespace/$/function/list", {"include_definition": "true"}),
         (
@@ -1909,6 +1917,9 @@ async def test_async_remote_list_functions_returns_typed_versions():
         functions = [item async for item in db.list_functions()]
 
     assert functions == [created]
+    assert functions[0].source_digest == (
+        normalize_score.registration_request.artifact.digest
+    )
     assert [path for path, _ in state["requests"]] == [
         "/v1/namespace/$/function/list",
         "/v1/namespace/$/function/list",