feat: add toLower/toUpper support, fix type coercion error

[youssef-tharwat]

Summary

• Add support for toLower/toUpper (Cypher) and lower/upper (SQL) string functions
• Change fallback for unsupported functions from lit(0) to ScalarValue::Null to prevent type coercion errors

Problem

When using toLower(s.name) CONTAINS 'x' with integer columns in the RETURN clause, a type coercion error occurred:

type_coercion: There isn't a common type to coerce Int32 and Utf8 in LIKE expression

This happened because toLower was not implemented and fell through to the default case which returned lit(0) (Int32). When combined with CONTAINS (translated to LIKE), DataFusion couldn't coerce Int32 to Utf8.

Solution

1. Implemented toLower/toUpper using DataFusion's lower()/upper() functions
2. Changed the fallback for unsupported functions from lit(0) to ScalarValue::Null, which coerces to any type in SQL semantics

Test plan

• Added unit tests for toLower, toUpper, lower, upper functions
• Added integration test reproducing the exact bug scenario
• All 196 existing tests pass

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.24731% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...t/lance-graph/src/datafusion_planner/expression.rs	89.24%	10 Missing ⚠️

📢 Thoughts on this report? Let us know!

[ChunxuTang]

@youssef-tharwat Thanks so much for the fix!
I just left very minor comments for the tests. The fix generally looks good to me.

Meanwhile, could you fix the style check error of your PR? Feel free to tag me when you want me to review again.

[youssef-tharwat]

@ChunxuTang all done :)

[ChunxuTang]

LGTM! Thanks for the contribution.

[ChunxuTang]

@youssef-tharwat One minor thing: there's a style check error reported by the GitHub workflows. Could you run cargo fmt --manifest-path Cargo.toml locally to fix the error and update your PR?