bug(openai): _create_usage_metadata crashes with TypeError when service_tier is set and cached_tokens is missing

### Checked other resources - [x] This is a bug, not a usage question. - [x] I added a clear and descriptive title that summarizes this issue. - [x] I used the GitHub search to find a similar question and didn't find it. - [x] I am sure that this is a bug in LangChain rather than my code. - [x] The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package). - [x] This is not related to the langchain-community package. - [x] I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS. ### Package (Required) langchain-openai ### Describe the bug Found this while reading the service_tier token arithmetic in `_create_usage_metadata`. When `service_tier` is "priority" or "flex" and the API response doesn't include `cached_tokens` in `prompt_tokens_details`, the subtraction crashes: ```python from langchain_openai.chat_models.base import _create_usage_metadata _create_usage_metadata( {"prompt_tokens": 100, "completion_tokens": 50, "total_tokens": 150, "prompt_tokens_details": {}, "completion_tokens_details": {}}, service_tier="priority" ) # TypeError: unsupported operand type(s) for -: 'int' and 'NoneType' ``` The dict stores `None` for missing `cached_tokens` via `.get("cached_tokens")`, then `.get(key, 0)` returns `None` (not 0) because the key exists with value None. Same bug in `_create_usage_metadata_responses`. Related to #36500 which fixed the same pattern in other token paths but missed the service_tier arithmetic. ### System Info langchain-openai 1.1.12, Python 3.12