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-core/tests/duplicate_detection_test.rs

@@ -13,6 +13,7 @@ fn chunk(content: &str, finished: bool) -> CompletionChunk {
         finished,
         tool_calls: None,
         usage: None,
+        stop_reason: None,
     }
 }