refactor: tighten spark_functions macros via expr_fn

Switch most spark wrappers from UDF-direct path (which forced
`spark_udf_fixed!(name, fn_category::name, args...)` repetition) to a
`spark_expr_fn!` macro that mirrors the existing `expr_fn!` macro in
`functions.rs`, so calls collapse to `spark_expr_fn!(sha2, arg1
bit_length);`.

UDF-direct retained for genuinely variadic functions whose upstream
`expr_fn` wrappers were generated with a single-`Expr` arm by
`export_functions!` (concat, array, xxhash64, parse_url family, etc.) so
that the Python side keeps its `*args` ergonomics.

Aggregates collapse the same way via `spark_aggregate!` mirroring
`aggregate_function!`. Net 173 lines removed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer