fix(core): Use WaitStrategy internally for wait_for function

[pmorch]

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 stating core should no longer warn, and jonaslb's reply 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

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.