[PR] Deduplicate ASCII and char paths in levenshtein_distance_with_threshold [datafusion-comet]

["EyobelG (via GitHub)" <gi...@apache.org>]

EyobelG opened a new pull request, #7154:
URL: https://github.com/apache/datafusion-comet/pull/7154

   ## Summary
   - `levenshtein_distance_with_threshold` had the same banded dynamic-programming algorithm duplicated for the ASCII (`u8`) fast path and the Unicode (`char`) path, differing only in element type and variable names.
   - Extracted the shared logic into a generic `banded_levenshtein<T: PartialEq + Copy>(s: &[T], t: &[T], threshold: i32) -> i32` helper, so band bounds, out-of-band handling and threshold capping only need to be maintained once.
   - The `is_ascii` fast path is preserved: ASCII input still goes through the helper via `s.as_bytes()`/`t.as_bytes()`, so no `char`-vector allocation is introduced for that case. Non-ASCII input still collects into `Vec<char>` before calling the helper, as before.
   - No logic changed beyond the extraction — control flow, bounds, and arithmetic are identical to the two prior copies, just parameterized over `T`.
   
   Fixes #6980.
   
   ## Test plan
   - Traced the extracted helper against both the ASCII and Unicode call sites to confirm it's byte-for-byte equivalent to the two original code paths (only renamed `s_bytes`/`s_chars` → `s`, `t_bytes`/`t_chars` → `t`, `t_byte`/`t_char` → `t_elem`).
   - Did not have a local Rust toolchain available to run `cargo test` in this environment — relying on CI to run the existing suite (`test_levenshtein_distance_with_threshold`, `test_levenshtein_distance_with_threshold_max_int`, `test_levenshtein_distance_with_threshold_unicode`, `test_longer_inputs_correctness`, etc. in `levenshtein.rs`).


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