standard-tests: refactoring and fixes (#31703)
- `libs/core/langchain_core/messages/base.py`: add model name to
examples [per
docs](https://python.langchain.com/api_reference/standard_tests/integration_tests/langchain_tests.integration_tests.chat_models.ChatModelIntegrationTests.html#langchain_tests.integration_tests.chat_models.ChatModelIntegrationTests.test_usage_metadata)
("0.3.17: Additionally check for the presence of model_name in the
response metadata, which is needed for usage tracking in callback
handlers")
- `libs/core/langchain_core/utils/function_calling.py`: correct typo
-
`libs/standard-tests/langchain_tests/integration_tests/chat_models.py`:
- `magic_function(input)` -> `magic_function(_input)` to prevent warning
about redefining built in `input`
- relocate a few tests for better grouping and narrative flow
- suppress some type hint warnings following suit from similar tests
- fix a few more typos
- validate not only that `model_name` is defined, but that it is not
empty (test_usage_metadata)
