test: add regression tests for beyond-count and off-by-one settlement listing boundaries

[onyekachi66]

Closes #150

Overview

This PR addresses a test-coverage gap identified in the settlement pagination logic. Specifically, the boundary where the paginated request parameter start exceeds the total number of settlements (settlement_count()) did not have explicit, dedicated coverage to prevent regression panics.

While the existing implementation correctly handles this under-the-hood (using loop guards like id <= count with (out.len() as u32) < limit), this PR guarantees safety at these boundaries for external integrators who might naively increment start beyond the available index set.

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Changes Made

Tests Added in src/test.rs

Added two new unit tests to explicitly lock down the behavior of the following four APIs:

1. list_settlements
2. list_settlements_by_anchor
3. list_settlements_by_asset
4. list_settlements_by_status

1. test_list_settlements_beyond_count

• Scenario: Checks pagination behavior when the start parameter is set significantly higher than the total number of settlements.
• Verification: Initializes an environment with two active settlements and queries all four endpoints with a start value of 100. Asserts that all endpoints successfully return an empty Vec rather than throwing an error or panicking.

2. test_list_settlements_off_by_one_boundary

• Scenario: Targets the critical off-by-one boundary where the start parameter is exactly equal to settlement_count() + 1.
• Verification: Initializes an environment with two active settlements (IDs 1 and 2, total settlement_count() = 2). Queries all four endpoints with start set to 3. Asserts that each call returns an empty Vec without panic.

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Acceptance Criteria Checklist

• Tested list_settlements beyond-count boundary.
• Tested list_settlements_by_anchor beyond-count boundary.
• Tested list_settlements_by_asset beyond-count boundary.
• Tested list_settlements_by_status beyond-count boundary.
• Explicitly verified the start == count + 1 off-by-one boundary.
• Ensured no panics or errors occur at these boundaries.