Add missing string functions (contains, instr, position, substring_index)

[timsaucer]

Summary

A few string functions available in upstream DataFusion are not yet exposed in datafusion-python.

Missing Functions

• contains — checks if a string contains a substring
• instr — returns the position of a substring within a string
• position — returns position of substring (SQL standard syntax)
• substring_index — returns the substring before/after N occurrences of a delimiter

Upstream Reference

• https://datafusion.apache.org/user-guide/sql/scalar_functions.html (String Functions section)

Implementation

• Rust bindings: crates/core/src/functions.rs
• Python wrappers: python/datafusion/functions.py

Note: This gap analysis was performed using an AI agent comparing upstream DataFusion v53 documentation against the current datafusion-python codebase.

[timsaucer]

Ok, there was a failure in the skill used to generate this PR. Specifically, contains, instr, and position do exist in the Python API just not in the rust bindings because they have aliases. That is correct. The only one we need to add is substring_index.