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/databricks.rs

@@ -493,6 +493,7 @@ impl DatabricksProvider {
                                 finished: false,
                                 usage: None,
                                 tool_calls: None,
+                                stop_reason: None,
                             };
                             if tx.send(Ok(text_chunk)).await.is_err() {
                                 debug!("Receiver dropped");