feat: inline encoding for Python aggregate and window UDFs (2/4)

[timsaucer]

Which issue does this PR close?

No associated issue. PR 2 of 4 stacked on #1544. The diff shown against `main` is cumulative until #1544 merges — review the commits on `pr2-agg-window-inline` directly, or wait for #1544 to merge for a clean diff.

Rationale for this change

PR 1 closed the round-trip for scalar UDFs. The same shipped-expression problem applies to Python aggregate and window UDFs: their accumulator / partition-evaluator factory is a Python callable, so a receiver that only has the UDF name cannot reconstruct one. This PR extends the inline-encoding mechanism so the natural `pickle.dumps(expr)` pattern also works for expressions referencing Python UDAFs and UDWFs.

What changes are included in this PR?

Codec extension is a straight parallel of the scalar path. New wire-format families:

Kind	Family magic	Cloudpickle tuple shape
Agg	`DFPYUDA`	`(name, accumulator_factory, input_schema_bytes, return_schema_bytes, state_schema_bytes, volatility_str)`
Window	`DFPYUDW`	`(name, evaluator_factory, input_schema_bytes, return_schema_bytes, volatility_str)`

The aggregate state schema is encoded as a full IPC schema (not a positional `Vec`), so the post-decode UDF reports the same names, nullability, and metadata as the sender. This matters for accumulators whose `StateFieldsArgs` consumers key off names rather than positions.

To let the codec downcast and grab the Python callable directly, two existing UDF impls are restructured:

• `udaf.rs`: introduces a named `PythonFunctionAggregateUDF` that stores the `Py` accumulator factory. `PyAggregateUDF.new` now wires `AggregateUDF::new_from_impl(PythonFunctionAggregateUDF::new(...))` instead of the prior `create_udaf` + closure path. State field names default to synthesized `state_{i}` on the Python constructor path; `from_parts` (called by the decoder) restores the full schema from the IPC payload.
• `udwf.rs`: renames `MultiColumnWindowUDF` → `PythonFunctionWindowUDF` and drops the `PartitionEvaluatorFactory` + `PtrEq` wrapping. Stores the `Py` evaluator directly. `PartialEq` / `Hash` pick up the same pointer-identity fast path and `eq` exception-logging behavior as `PythonFunctionScalarUDF` in PR 1.

User-facing surface:

• `AggregateUDF.name` and `WindowUDF.name` properties (parallel to `ScalarUDF.name` from PR 1).
• Existing UDAF/UDWF construction paths are unchanged at the user level — same constructors, same arguments, same semantics. The internal impl swap is invisible.

Are there any user-facing changes?

• Python aggregate and window UDFs survive `pickle.dumps` / `pickle.loads` and `Expr.to_bytes` / `Expr.from_bytes` round-trips. The decoded UDF reproduces the original state schema and runs end-to-end (verified by `test_agg_udf_evaluates_after_roundtrip`, which aggregates over a 5-row frame after a pickle round-trip).
• `AggregateUDF.name` and `WindowUDF.name` are new public properties.
• `MultiColumnWindowUDF` is renamed to `PythonFunctionWindowUDF`. The struct was `pub` but only used within the crate; no in-tree caller breaks. Downstream Rust users importing it directly would need to update.

The `MultiColumnWindowUDF` rename is a Rust-side breaking change, so adding `api change` even though no Python-facing API breaks.