release: 2.9.1 (#514)

* empty-properties: improved docs and error message.

* chore(internal): add tests for breaking change detection

* chore: updated readme link

* release: 2.9.1

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Co-authored-by: D Gardner <damien@gardnerhrm.com>
Co-authored-by: stainless-app[bot] <142633134+stainless-app[bot]@users.noreply.github.com>
Co-authored-by: Kevin Whinnery <kwhinnery@openai.com>
Co-authored-by: Tomer Aberbach <tomer@stainless.com>