backlog: Remove text completion provider deprecations in 0.23.0

[Pouyanpi]

Target release: 0.23.0

What to remove

1. register_llm_provider: remove function from nemoguardrails/llm/providers/__init__.py
2. get_llm_provider_names: remove function from nemoguardrails/llm/providers/__init__.py
3. LangChainFramework.register_llm_provider: remove method from nemoguardrails/integrations/langchain/llm_adapter.py
4. LangChainFramework.get_llm_provider_names: remove method from nemoguardrails/integrations/langchain/llm_adapter.py
5. _llm_providers dict: remove from nemoguardrails/integrations/langchain/providers/providers.py
6. _discover_langchain_community_llm_providers(): remove from nemoguardrails/integrations/langchain/providers/providers.py
7. _patch_acall_method_to(): remove (only used for llm providers)
8. _get_text_completion_provider(): remove from nemoguardrails/integrations/langchain/providers/providers.py
9. _init_text_completion_model(): remove from nemoguardrails/integrations/langchain/langchain_initializer.py
10. CLI warning suppression: remove warnings.catch_warnings blocks from nemoguardrails/cli/providers.py, collapse to single provider list
11. mode parameter: remove from init_llm_model() and LangChainFramework.create_model() kwargs handling
12. Test cleanup: remove test_discover_langchain_community_llm_providers and related tests from tests/llm/test_version_compatibility.py

Tests to update

• tests/test_custom_llm.py::test_custom_llm_registration: remove or convert to chat provider test
• tests/llm/test_frameworks.py::TestProviderRegistration: remove llm provider tests, remove clean_providers fixture llm backup
• tests/llm/test_version_compatibility.py: remove llm provider discovery tests
• tests/test_configs/with_custom_llm/: convert to chat model or remove