Fix tests for openai v2.45.0 cache_write_tokens compatibility

[fanzeyi]

Problem

CI started failing for the langchain tests and type checks due to a breaking change in openai v2.45.0 (released July 9, 2026).

openai v2.45.0 added a new required field cache_write_tokens: int to InputTokensDetails in openai.types.responses.response_usage:

# v2.44.0
class InputTokensDetails(BaseModel):
    cached_tokens: int

# v2.45.0
class InputTokensDetails(BaseModel):
    cache_write_tokens: int   # NEW, required, no default
    cached_tokens: int

Existing tests construct InputTokensDetails(cached_tokens=...) without cache_write_tokens, causing ValidationError on v2.45.0.

Failures

This causes 6 test failures and 4 ty type-check errors:

• test_convert_responses_usage_to_usage_metadata (2 parametrized)
• test_extract_response_usage_from_chunk (2 parametrized)
• test_build_usage_chunk_from_responses (1 parametrized)
• test_chat_databricks_responses_api_invoke_returns_usage_metadata

The langchain_test and langchain_test_with_extras jobs fail with uv: highest resolution (which picks up v2.45.0), while lowest-direct still passes (uses older version).

Fix

Pass cache_write_tokens=0 to all InputTokensDetails constructions.

This is backward compatible: openai's BaseModel uses extra='allow', so the extra field is silently accepted on older versions (e.g. v2.44.0) that don't define it.

Verified passing on both openai==2.44.0 and openai==2.45.0.