fix: honor static cache max len config

[he-yufeng]

Summary

• honor generation_config.cache_config["max_cache_len"] when auto-preparing static caches
• keep the cache length at least as large as the current generation requires
• add a tiny GPT-2 regression test that checks the prepared StaticCache size

Fixes #46424.

To verify

• PYTHONPATH=src python -m pytest tests\generation\test_utils.py::GenerationIntegrationTests::test_static_cache_uses_max_cache_len_from_cache_config -q
• python -m py_compile src\transformers\generation\utils.py tests\generation\test_utils.py
• git diff --check