Port internal/testutil/wait polling helpers

[osvaldoandrade]

Add polling helpers to replace time.Sleep in tests.

Goal: Deterministic, fast tests with clear failure messages.

Implementation:

• Create internal/testutil/wait/
• Implement Eventually(t, cond func() bool, timeout, interval, msg)
• Implement Never(t, cond func() bool, duration)
• Replace time.Sleep + assertion patterns in tests

Acceptance criteria:

• Helpers available
• Example usage in test
• Tests are faster and less flaky

Part of: #654
Requires: #645 (foundations)