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.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.59%. Comparing base (2d9eee3) to head (0d1cee7).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
core/testcontainers/core/waiting_utils.py	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #942      +/-   ##
==========================================
- Coverage   78.18%   77.59%   -0.59%     
==========================================
  Files          14       14              
  Lines        1210     1214       +4     
  Branches      187      187              
==========================================
- Hits          946      942       -4     
- Misses        222      231       +9     
+ Partials       42       41       -1     

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