fix: use exclude_unset for SSE streaming chunk serialization

exclude_none was too aggressive — it stripped finish_reason: null from
intermediate chunks (which OpenAI includes). exclude_unset correctly
omits only fields never passed to the constructor (like reasoning_content
on content-only chunks) while preserving explicitly-set finish_reason: null.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "tatlock"
-version = "2.0.5"
+version = "2.1.0"
 description = "OpenAI-compatible API with Ollama backend"
 requires-python = ">=3.12"
 dependencies = [