Add typed Protocols for FFI capsule exports (part of #1577)

[stantheman0128]

Which issue does this PR close?

Part of #1577. This PR covers 3 of the 6 items in that umbrella issue (see "Not included" below for why the rest are left out).

Rationale for this change

The FFI-pipeline typing audit referenced in #1577 found several places where DataFusion's FFI types are already imported on the Rust side, but the Python surface has no typed Protocol describing the expected PyCapsule dunder method. Callers are left to read the Rust source or guess the signature.

What changes are included in this PR?

Adds three Protocol classes, following the existing TableProviderExportable / PhysicalOptimizerRuleExportable pattern (name, docstring, and single dunder method), and points the corresponding parameter/return type hints at them:

• datafusion.user_defined.TableFunctionExportable: describes the __datafusion_table_function__(self, session) PyCapsule method already duck-typed via hasattr in TableFunction.__init__ / TableFunction.udtf (those hasattr checks are the actual runtime dispatch and are left untouched). The func parameter on TableFunction.__init__ is now typed as Callable[..., Any] | TableFunctionExportable, and a new udtf overload documents the FFI-capsule call shape.
• datafusion.context.ExtensionOptionsExportable: describes __datafusion_extension_options__(self), required by SessionConfig.with_extension (see crates/core/src/context.rs, SessionConfig::with_extension). with_extension's extension parameter is now typed against it.
• datafusion.context.TaskContextProviderExportable: describes __datafusion_task_context_provider__(self). SessionContext already exposes one of these in context.py, and this protocol documents the shape for other extensions that want to decode or produce one, matching the task_context_from_pycapsule helper already in crates/util/src/lib.rs.

Not included from #1577's item list

The remaining 3 items are execution-engine-level questions rather than typing changes, so I split each into its own issue for design discussion:

• FFI_TableProvider, TableProvider(ABC) (item 2): wiring a plain-Python subclass through to the Rust TableProvider trait needs a new RustWrappedPyTableProvider. Design discussion: Wire FFI_TableProvider / TableProvider(ABC) through to the Rust TableProvider trait #1668
• FFI_TableProviderFactory, from_pycapsule helper (item 5): the call site this item points at passes an argument to the dunder method, which the existing from_pycapsule! / try_from_pycapsule! macros don't support (call0() only). Discussion: from_pycapsule! / try_from_pycapsule! macros don't support dunder methods that take arguments #1669
• WindowUDF, ABC equivalent to Accumulator (item 6): WindowEvaluator already provides this capability; it's just not ABCMeta-shaped because the required evaluate* method depends on a flag matrix. Confirming whether the literal ABC form is still wanted: WindowEvaluator already provides the pure-Python window UDF base class #1577 (item 6) asks for #1670

Are there any user-facing changes?

No runtime behavior changes. This only adds type hints (new Protocol classes) and updates existing parameter/return annotations to reference them.

Validation run locally:

• uvx ruff@0.15.1 check python/datafusion/user_defined.py python/datafusion/context.py - passed
• uvx ruff@0.15.1 format --check python/datafusion/user_defined.py python/datafusion/context.py - passed
• git diff --check - passed
• python -m py_compile on both files - passed