fix(core): Use WaitStrategy internally for wait_for function (#942)

# fix(core): Use WaitStrategy internally for wait_for function

## Summary

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

This fixes the import-time deprecation warning from
#874.

See [alexanderankin's
comment](#874 (comment))
stating core should no longer warn, and [jonaslb's
reply](#874 (comment))
identifying that `wait_for` is still decorated with
`@wait_container_is_ready` at module level, so it generates a warning.

## Problem

The `wait_for` function was decorated with `@wait_container_is_ready()`,
which emits a deprecation warning at **decoration time** (import time),
not when the function is called. This caused warnings even when users
never used `wait_for`.

### Reproducer

```bash
mkdir /tmp/reproduce && cd /tmp/reproduce
echo 'import testcontainers.core.waiting_utils
def test_import(): pass' > test_import.py
uv run --with testcontainers --with pytest pytest test_import.py
```

```
=============================== warnings summary ===============================
waiting_utils.py:215: DeprecationWarning: The @wait_container_is_ready decorator
  is deprecated and will be removed in a future version...
    @wait_container_is_ready()

1 passed, 1 warning
```

Note: `wait_for` is never called - the warning is emitted just by
importing the module.

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"}