Fix panic on multi-byte chars in filter_json buffer truncation

The buffer truncation code was slicing at a raw byte offset which could
land in the middle of a multi-byte character (like emojis), causing a
panic. Fixed by using char_indices() to find valid character boundaries.

Also added stop_reason field to CompletionChunk initializers in tests
to complete the stop_reason feature addition.

- Fix byte boundary panic in filter_json.rs line 327
- Add test for multi-byte character handling
- Update test files with missing stop_reason field

crates/g3-providers/src/openai.rs

@@ -157,6 +157,7 @@ impl OpenAIProvider {
                                         finished: true,
                                         tool_calls,
                                         usage: accumulated_usage.clone(),
+                                        stop_reason: None, // TODO: Extract from OpenAI response
                                     };
                                     let _ = tx.send(Ok(final_chunk)).await;
                                 }