Guard detection, guard-aware exact role checks, and validated Role::users relation

[imhayatunnabi]

Issues resolved for guard checking:

• only treat the first Gate argument as a guard when it matches a configured guard name. Prevents misinterpreting arbitrary strings (eg, policy params) as guards.
• Compare role counts scoped by the provided $guard (when present), avoiding false negatives when the model has extra roles on other guards.
• Validate that the role’s guard_name maps to a configured provider model. If not, throw a clear InvalidArgumentException instead of constructing an invalid relation.

Testcase: Polyfill for older Orchestra Testbench versions in prefer-lowest CI matrix that expect a static $latestResponse property on the base TestCase. Newer versions manage this internally; declaring it here is harmless.