test(splitter): add boundary tests for splits_of_paged (#325)

[Hollujay]

Description

splits_of_paged had no tests. Pagination logic is exactly the kind of code that silently breaks at its boundaries. This PR adds four test cases covering all edge cases identified in #325.

Changes

Added 4 tests to contracts/splitter/src/test.rs:

• splits_of_paged_start_beyond_list_length — verifies that a start index equal to or greater than the list length returns an empty page
• splits_of_paged_limit_zero — verifies that a limit of 0 returns an empty page regardless of start
• splits_of_paged_runs_past_end — verifies that when fewer items remain than the requested limit, the page correctly stops at the end of the list
• splits_of_paged_lands_exactly_on_last_item — verifies that a start + limit landing exactly on the last item returns that single item

CI

• cargo fmt — clean
• cargo clippy --all-targets -- -D warnings — clean
• cargo test — all 68 tests pass

Closes #325