fix: make delta field optional in ChunkChoice

The final streaming chunk from some providers contains only finish_reason
and usage, with no delta field. Made it optional to prevent parse errors.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: 2 people

rust/crates/api/src/providers/openai_compat.rs

@@ -790,6 +790,7 @@ struct ChatCompletionChunk {
 
 #[derive(Debug, Deserialize)]
 struct ChunkChoice {
+    #[serde(default)]
     delta: ChunkDelta,
     #[serde(default)]
     finish_reason: Option<String>,