Fix PoolConstraints infinite wait when the max value Is zero

[powerumc]

I've created issue #397 and am submitting a PR that adds effective tests.
In summary, if you set the pool size using PoolConstraints::new(0, 0), the connection will wait indefinitely.

I've added two tests to verify this behavior at runtime and at compile time:

• PoolConstraints::new(0, 0);
• PoolConstraints::new_const::<0, 0>();

Please review. Thank you.

[powerumc]

@blackbeam Hi there 👋

I hope you’re doing well. I just wanted to kindly follow up on this PR and would greatly appreciate your review whenever you have a moment. Please let me know if there’s anything I can clarify or improve. Thank you!