release: 3.18.0 (#322)

* chore(internal): update gitignore

* feat: Add explicit SSE event names for local v3 streaming

* feat: Include LLM headers in ModelConfig

* chore(ci): skip lint on metadata-only changes

Note that we still want to run tests, as these depend on the metadata.

* release: 3.18.0

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Co-authored-by: stainless-app[bot] <142633134+stainless-app[bot]@users.noreply.github.com>