Add custom sleep handler for testability

[mariomka]

Saloon sleeps with a raw usleep() in two places: the retry loop in SendsRequests and DelayMiddleware. These run even when requests are mocked with MockClient, so any test covering retries or delays pays real wall-clock time and there's no seam to avoid it. In one of our apps, a test file went from 9.5s to 4.4s once sleeps were skipped.

This PR adds a sleep seam on Config, mirroring Config::setSenderResolver():

// In a test bootstrap for skipping all sleeps
Config::sleepUsing(fn () => null);

// Restore the default behaviour
Config::sleepUsing(null);

Both call sites now route through Config::sleep($microseconds), which falls back to usleep() when no handler is set.

As a follow-up, the Laravel plugin could bind the handler to Illuminate\Support\Sleep so Sleep::fake() works out of the box, happy to open that PR too.

I have considered some alternatives but I went with a closure on Config because it mirrors setSenderResolver(), but happy to rework it if you prefer another shape:

• A Sleeper contract resolved like the sender, for an object-based seam instead of a closure.
• A standalone helper (e.g. Saloon\Helpers\Sleep::for($microseconds)) with a static test override, keeping Config untouched.

[Sammyjo20]

Amazing PR @mariomka thank you!

[mariomka]

@Sammyjo20 would you be interested in a follow‑up PR for the Laravel plugin to use Illuminate\Support\Sleep for sleeping, so that Sleep::fake() works out of the box?

[Sammyjo20]

Absolutely @mariomka yes please! Does this mean that anyone using the Laravel plugin will automatically get access to Sleep::fake() etc?