Support Cast node in pushdown logic

[evertlammerts]

Related: #98

When the decimal precision is anything other than 38, Polars wraps the expression in a Cast node. _pl_tree_to_sql didn't handle Cast nodes, causing silent pushdown failure. This fix makes sure these nodes get pushed down as well.

import duckdb
import polars as pl
import json

# Create a decimal column with precision != 38
con = duckdb.connect()
rel = con.sql("SELECT a::DECIMAL(20,0) AS a FROM range(10) AS t(a)")

lazy_df = rel.pl(lazy=True)

# This filter works but pushdown silently fails
result = lazy_df.filter(pl.col("a") == 1).collect()
print(f"Result: {len(result)} rows")  # Returns 1 row (correct)

# Polars serialized it as follows
expr = pl.col("a") == 1
tree = json.loads(expr.meta.serialize(format="json"))
print(json.dumps(tree, indent=2))
# Contains: "Cast": {"expr": ..., "dtype": {"Decimal": [20, 0]}, ...}

Note that we have to also check for the strictness of the cast node:

• NonStrict casts are pushable, they are auto-inserted type coercion casts (like DECIMAL(20,0) to DECIMAL(38,0))
• Strict casts are fallible (and can error, see fix: Fix predicate pushdown of fallible expressions pola-rs/polars#22669)