fix: make file content part fields optional and add file_id

[robert-j-y]

Description

Makes filename and file_data fields optional in ChatCompletionContentPartFile and adds file_id field for OpenAI file uploads support. This aligns the type with the API spec.

Checklist

• I have run pnpm stylecheck and pnpm typecheck
• I have run pnpm test and all tests pass
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)

Changeset

• I have run pnpm changeset to create a changeset file

Note: A changeset is required for your changes to trigger a release. If your PR only contains docs, tests, or CI changes that don't need a release, run pnpm changeset --empty instead.

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Note: Tests and documentation are not applicable since this is a type-only change that makes fields optional (non-breaking).