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.

Start by comparing the existing TableProviderExportable, CatalogProvider, SchemaProvider, Accumulator, and from_pycapsule! implementations with the named entry points in user_defined.py, context.rs, SessionConfig.with_extension, and PySessionContext.__datafusion_task_context_provider__. Trace each listed FFI type and its existing Rust import or capsule path. Done means the six missing typed interfaces, helper, and example are covered consistently and the relevant tests pass.

Written by the indexing model from the issue text.