Fix distinguishing the joined geospatial types for ST_KNN

[Kontinuation]

This PR fixes a regression introduced by #57, which added geography type checking to prevent optimized spatial joins from being used with unsupported geography types.

The problematic patch incorrectly matches left and right expressions in KNNPredicate to query plans. Actually, how left and right maps to the original query plans depends on the value of probe_side field. This is quite misleading and may be addressed in a future PR by renaming left and right to build and probe.

The function for checking if geospatial types are supported now returns Result<bool> to propagate errors during the field type extraction and matching process. This prevents silent matching failures due to bugs from happening.

[Copilot]

Pull Request Overview

This PR fixes a regression in geospatial type checking for ST_KNN operations by correctly mapping left/right expressions to query plans based on the probe_side field. The previous implementation incorrectly assumed a direct mapping between left/right expressions and schema positions.

• Changes error handling from boolean returns to Result types for better error propagation
• Adds proper probe_side consideration when validating KNN predicates to ensure correct schema-to-expression mapping
• Includes comprehensive test coverage for the fixed KNN predicate validation logic

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[paleolimbot]

Thank you!