refactor(rust/sedona-spatial-join): Unify execute and execute_knn, move end-to-end tests in exec.rs to tests directory

[Kontinuation]

Summary

• relocate spatial join integration tests from exec.rs into rust/sedona-spatial-join/tests/
• keep test helpers and data builders intact while switching to public crate imports
• preserve existing behavior while enabling integration-style execution

[Copilot]

Pull request overview

This PR refactors SpatialJoinExec::execute to unify KNN and non-KNN execution paths, and relocates end-to-end spatial join tests from exec.rs into an integration test under tests/.

Changes:

• Unifies execute logic for KNN and non-KNN predicates by selecting build/probe plans dynamically.
• Removes the KNN-specific execute_knn implementation and shares component initialization + ordering logic.
• Moves the large test suite out of src/exec.rs into rust/sedona-spatial-join/tests/spatial_join_integration.rs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
rust/sedona-spatial-join/src/exec.rs	Unifies KNN + non-KNN execution logic and deletes inline unit tests from this module.
rust/sedona-spatial-join/tests/spatial_join_integration.rs	Adds integration-style tests that exercise optimizer behavior and execution end-to-end.

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