fix(python): preserve code tokenizer filter defaults (#4370)

Python's FTS bindings always forwarded `stem=True` and
`remove_stop_words=True`, overriding Lance's code tokenizer defaults. As
a result, code indexes stemmed identifiers and discarded stop words, and
`lancedb.tokenize` reproduced the same behavior.

Use `None` for omitted filter options and only apply explicit booleans
in the Rust bindings. This preserves tokenizer defaults across local and
remote index creation, the deprecated `create_fts_index` helpers, and
standalone tokenization. Update the signatures and documentation to
describe the inherited defaults.

Regression tests cover stored local settings, token output, stop-word
searches, remote request payloads, explicit overrides, and the existing
simple tokenizer behavior. The regression suite produced 16 failures
before the fix and passes all 28 cases after it.

Validation:

- Rebuilt the Python extension using the uv-managed development
environment.
- Full Python FTS and index test files plus relevant remote index tests:
105 passed.
- `cargo clippy --quiet --features remote --tests --examples -p
lancedb-python`
- `cargo fmt --all`, Ruff format/lint with CI's pinned version 0.9.9,
and Pyright (zero errors).

Fixes #4365

<!-- lance-gatekeeper-fix:v1 agent=9c1db2d3fc25c49c5b340f8debf43a0f
generation=1 -->

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>

python/src/index.rs

@@ -54,8 +54,6 @@ pub fn extract_index_params(source: &Option<Bound<'_, PyAny>>) -> PyResult<Lance
                     .with_position(params.with_position)
                     .lower_case(params.lower_case)
                     .max_token_length(params.max_token_length)
-                    .remove_stop_words(params.remove_stop_words)
-                    .stem(params.stem)
                     .ascii_folding(params.ascii_folding)
                     .ngram_min_length(params.ngram_min_length)
                     .ngram_max_length(params.ngram_max_length)
@@ -65,6 +63,12 @@ pub fn extract_index_params(source: &Option<Bound<'_, PyAny>>) -> PyResult<Lance
                         DocumentGranularity::try_from(params.document_granularity.as_str())
                             .map_err(|err| PyValueError::new_err(err.to_string()))?,
                     );
+                if let Some(stem) = params.stem {
+                    inner_opts = inner_opts.stem(stem);
+                }
+                if let Some(remove_stop_words) = params.remove_stop_words {
+                    inner_opts = inner_opts.remove_stop_words(remove_stop_words);
+                }
                 if let Some(memory_limit) = params.memory_limit {
                     inner_opts = inner_opts.memory_limit_mb(memory_limit);
                 }
@@ -215,8 +219,8 @@ struct FtsParams {
     language: String,
     max_token_length: Option<usize>,
     lower_case: bool,
-    stem: bool,
-    remove_stop_words: bool,
+    stem: Option<bool>,
+    remove_stop_words: Option<bool>,
     custom_stop_words: Option<Vec<String>>,
     ascii_folding: bool,
     ngram_min_length: u32,