test(router-access): cover set_role_limits and get_role_limits (#1047)

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Summary

set_role_limits (contracts/router-access/src/lib.rs lines 93-111) and get_role_limits (lines 113-119) are admin-facing configuration entry points for the (max_roles, max_grants_per_role) policy that governs MaxRolesExceeded / MaxGrantsPerRoleExceeded enforcement elsewhere in the contract. Both functions had zero direct test coverage: a grep for the two names in the test module returns no matches, and the issue confirms it. A regression in the 0-means-default logic or the super-admin auth gate could have shipped unnoticed.

Fix

This change adds the four tests the issue itself suggested, in the same style as the existing test_set_role_admin_* / test_grant_role_* cases:

• test_get_role_limits_defaults — before any set_role_limits call, the contract returns the compile-time defaults (100, 1_000).
• test_set_role_limits_updates_and_persists — a non-zero (max_roles, max_grants_per_role) is stored, and a subsequent get_role_limits returns the new values.
• test_set_role_limits_zero_resets_to_defaults — passing 0 for either field restores the compile-time default for that field.
• test_set_role_limits_unauthorized_fails — a non-super-admin caller is rejected with AccessError::Unauthorized.

I do not have a local Rust toolchain on this machine, so I could not run cargo test myself. The tests are written by analogy with the existing patterns in the same test module and use the existing setup() helper, so they should compile and pass without further changes. Please run the suite on a Soroban environment before merging.

Closes

#1047