Re-run validation and add admin-only limit change tests

[funds0033-cmyk]

Summary

This PR adds test coverage for the admin authorization requirements around set_limits, ensuring only authenticated administrators can modify contract limits and validating behavior before contract initialization.

Changes

• Added test_set_limits_requires_admin_auth to verify unauthorized calls fail and existing limits remain unchanged.
• Added test_set_limits_by_admin_succeeds to confirm authenticated administrators can successfully update contract limits.
• Extended test_admin_operations_require_initialization to verify set_limits returns NotInitialized before contract initialization.
• Updated docs/unit-tests.md with the new authorization test coverage.
• Updated docs/testing-guide.md to document the new admin authorization test scenarios.

Validation

• Verified unauthorized set_limits calls are rejected without modifying stored limits.
• Confirmed authenticated administrators can successfully update limits.
• Confirmed set_limits returns NotInitialized when invoked before contract initialization.
• Verified documentation reflects the new test coverage.

Closes #95