fix(core): Use WaitStrategy internally for wait_for function

Refactor the deprecated wait_for function to use the new WaitStrategy system
internally instead of the @wait_container_is_ready decorator.

The decorator emitted a deprecation warning at decoration time (import time),
causing warnings even when users never called wait_for.

Replace with an internal CallableWaitStrategy that uses WaitStrategy._poll()
for retry logic. This follows the intended migration path to the new system.

Fixes #874

Author: pmorch

core/testcontainers/core/waiting_utils.py

@@ -216,7 +216,6 @@ def wrapper(wrapped: Callable[..., Any], instance: Any, args: tuple[Any], kwargs
     return cast("Callable[[F], F]", wrapper)
 
 
-@wait_container_is_ready()
 def wait_for(condition: Callable[..., bool]) -> bool:
     warnings.warn(
         "The wait_for function is deprecated and will be removed in a future version. "
@@ -226,7 +225,15 @@ def wait_for(condition: Callable[..., bool]) -> bool:
         DeprecationWarning,
         stacklevel=2,
     )
-    return condition()
+
+    class CallableWaitStrategy(WaitStrategy):
+        def wait_until_ready(self, container: WaitStrategyTarget) -> None:
+            pass  # Required by ABC, but unused
+
+    strategy = CallableWaitStrategy()
+    if not strategy._poll(condition):
+        raise TimeoutError(f"Condition not satisfied within {strategy._startup_timeout}s")
+    return True
 
 
 _NOT_EXITED_STATUSES = {"running", "created"}