feat(python): add type-safe expression builder API (#3150)

Introduces col(), lit(), func(), and Expr class as alternatives to raw
SQL strings in .where() and .select(). Expressions are backed by
DataFusion's Expr AST and serialized to SQL for remote table compat.

Resolves: 
- https://github.com/lancedb/lancedb/issues/3044 (python api's)
- https://github.com/lancedb/lancedb/issues/3043 (support for filter)
- https://github.com/lancedb/lancedb/issues/3045 (support for
projection)

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

python/src/lib.rs

@@ -4,6 +4,7 @@
 use arrow::RecordBatchStream;
 use connection::{Connection, connect};
 use env_logger::Env;
+use expr::{PyExpr, expr_col, expr_func, expr_lit};
 use index::IndexConfig;
 use permutation::{PyAsyncPermutationBuilder, PyPermutationReader};
 use pyo3::{
@@ -21,6 +22,7 @@ use table::{
 pub mod arrow;
 pub mod connection;
 pub mod error;
+pub mod expr;
 pub mod header;
 pub mod index;
 pub mod namespace;
@@ -55,10 +57,14 @@ pub fn _lancedb(_py: Python, m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_class::<UpdateResult>()?;
     m.add_class::<PyAsyncPermutationBuilder>()?;
     m.add_class::<PyPermutationReader>()?;
+    m.add_class::<PyExpr>()?;
     m.add_function(wrap_pyfunction!(connect, m)?)?;
     m.add_function(wrap_pyfunction!(permutation::async_permutation_builder, m)?)?;
     m.add_function(wrap_pyfunction!(util::validate_table_name, m)?)?;
     m.add_function(wrap_pyfunction!(query::fts_query_to_json, m)?)?;
+    m.add_function(wrap_pyfunction!(expr_col, m)?)?;
+    m.add_function(wrap_pyfunction!(expr_lit, m)?)?;
+    m.add_function(wrap_pyfunction!(expr_func, m)?)?;
     m.add("__version__", env!("CARGO_PKG_VERSION"))?;
     Ok(())
 }