Pipeline FFI types: missing Protocols, ABCs, and capsule helpers #1577

Background

A v54 upstream coverage audit identified six FFI-pipeline gaps where DataFusion's FFI types are already imported on the Rust side but the Python surface is incomplete -- either missing a typed Protocol / ABC for users to implement against, or missing a from_pycapsule helper that the rest of the FFI surface uses. These are not v54-specific; they predate the v54 release. Filing this umbrella issue so the gaps are tracked together rather than disappearing into the audit report.

Items

1. FFI_TableFunction -- no TableFunctionExportable(Protocol). The duck-typed hasattr check at user_defined.py:1161,1205 should be replaced by a typed protocol mirroring TableProviderExportable.

2. FFI_TableProvider -- no Python TableProvider(ABC) analogous to CatalogProvider / SchemaProvider. Users implementing a custom table provider in Python have no abstract base class to subclass; the Exportable protocol is the only entry point.

3. FFI_ExtensionOptions -- no ExtensionOptionsExportable(Protocol). Currently consumed only at the SessionConfig.with_extension call site without a typed protocol describing the expected capsule export.

4. FFI_TaskContextProvider -- no Python Protocol and no example. Producer-only at PySessionContext.__datafusion_task_context_provider__; nothing imports one.

5. FFI_TableProviderFactory -- no from_pycapsule helper on RustWrappedPyTableProviderFactory. The capsule decode is inlined at context.rs:728-749 rather than going through the from_pycapsule! macro the other FFI capsule importers use.

6. WindowUDF -- no Python ABC equivalent to Accumulator for native Python window UDFs. Users can register a Rust-defined window UDF over FFI but cannot define one in pure Python.

Why deferred

Each item is small in isolation but they form a self-contained cleanup track. The audit grouped them so the work can be picked up together by a contributor focused on FFI ergonomics, rather than scattered across the v54 gap-closure PRs which focus on net-new surface area. Picking up any one item from this list is welcome.