feat: run length on binary input natively

[dwsmith1983]

Which issue does this PR close?

Closes #2348.

Rationale for this change

length(binary) currently falls back to Spark because Comet registered DataFusion's character_length, which accepts string types only. datafusion-spark now ships a Spark-compatible length that returns the byte count for binary and the character count for strings, so the fallback can go.

What changes are included in this PR?

• Register SparkLengthFunc in the native session under length, char_length, character_length and len, replacing character_length. The result is Int32 for every input width, matching Spark.
• Drop the BinaryType fallback from CometLength.
• A Rust roundtrip test through the Parquet schema adapter shows dictionary-typed string and binary columns are cast to Utf8/Binary before any expression sees them, so the replaced function's dictionary handling is not needed downstream. A Scala test reads a small Arrow-written Parquet file whose embedded schema declares dictionary-typed columns and checks length runs natively on it.
• Refresh the audit note in the string function audit doc and the stale notes in the bit_length/octet_length fixtures.

How are these changes tested?

• length.sql gains a binary table and queries covering multi-byte text, bytes that are not valid UTF-8, embedded NUL bytes, empty, NULL and all-NULL columns, binary reached through a struct field, an array element, a map value, unhex, substring and CAST(string AS BINARY), the Int32 result in arithmetic, a filter, a GROUP BY aggregate and after a native shuffle, and the char_length/character_length aliases. Every block asserts a fully native plan.
• CometStringExpressionSuite runs length on 1000 binary rows with and without dictionary encoding, and on the dictionary-typed file described above.
• A Rust test asserts length resolves to the Spark function returning Int32 for Utf8, LargeUtf8, Utf8View, Binary, LargeBinary and BinaryView with no cast inserted.
• Verified locally on Spark 3.5 and 4.0: CometSqlFileTestSuite length, CometStringExpressionSuite, cargo test, cargo clippy -D warnings, spotless.