fix(python): keep embedding functions registered without an alias readable - #4356

EmbeddingFunctionRegistry.register() documents that the class name is used when no alias is given, and it does register the class under that name. But it stored the alias itself (None) as __embedding_function_registry_alias__, which is what function_to_metadata() writes into the table schema. A table using such a function gets "name": null in its embedding_functions metadata, and any later read of the functions fails:

@get_registry().register()
class MyEmbed(TextEmbeddingFunction): ...

f = MyEmbed.create()
class Schema(LanceModel):
    text: str = f.SourceField()
    vector: Vector(4) = f.VectorField()

tbl = db.create_table("t", schema=Schema)
tbl.add([{"text": "hi"}])   # KeyError: None
tbl.search("hi")            # KeyError: None

This stores the key the class is registered under instead, so the class name round-trips. For aliased classes the key is the alias, so their metadata is unchanged. The TypeScript registry already falls back to ctor.name in the same way.

Added test_embedding_function_registered_without_alias, which creates a table with such a function, reopens it, and adds a row. It fails with KeyError before the change. test_embeddings.py, test_pydantic.py, test_table.py and test_db.py pass, and ruff format --check / ruff check are clean.