feat: represent registered Functions by OCI image identity (#4176)

Function versions identify independent Function objects and their
numeric revisions. Rust and Python expose the object ID, location,
canonical decimal version, metadata, and availability separately from
the OCI image digest. Computed-column applications carry the complete
object reference, so existing bindings retain their identity after a
name is removed and reused.

Source authoring keeps the existing
create_function/create_function_async, job wait, and column-binding
APIs. The server coordinates baking followed by registration; users do
not have to manage manifest digests to create a Function. The previous
stored Function representation is intentionally unsupported. Existing
contract tests and shared wire fixtures are migrated to the new model.

This SDK change accompanies the final integration layer
https://github.com/lancedb/sophon/pull/7887 in the Sophon stack:
https://github.com/lancedb/sophon/pull/7885 →
https://github.com/lancedb/sophon/pull/7886 →
https://github.com/lancedb/sophon/pull/7887. A metadata-only revision
can retain the same executable image; Function version numbers must not
be used as image cache keys.

---------

Co-authored-by: lancedb automation <robot@lancedb.com>
Co-authored-by: Yang Cen <bubble-cal@outlook.com>
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

docs/src/python/python.md

@@ -133,6 +133,8 @@ listing a storage directory.
 
 ::: lancedb.functions.PythonAdapterSpec
 
+::: lancedb.functions.FunctionImage
+
 ::: lancedb.functions.FunctionVersion
 
 ::: lancedb.functions.PythonRuntimeSpec