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

## 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.