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,
     }
 }
 

crates/g3-core/tests/incomplete_tool_call_test.rs

@@ -17,6 +17,7 @@ fn test_has_incomplete_tool_call_no_tool_pattern() {
         finished: false,
         tool_calls: None,
         usage: None,
+        stop_reason: None,
     };
     parser.process_chunk(&chunk);
     assert!(!parser.has_incomplete_tool_call());
@@ -30,6 +31,7 @@ fn test_has_incomplete_tool_call_complete_tool_call() {
         finished: false,
         tool_calls: None,
         usage: None,
+        stop_reason: None,
     };
     parser.process_chunk(&chunk);
     // Complete JSON should NOT be detected as incomplete
@@ -45,6 +47,7 @@ fn test_has_incomplete_tool_call_truncated_tool_call() {
         finished: false,
         tool_calls: None,
         usage: None,
+        stop_reason: None,
     };
     parser.process_chunk(&chunk);
     // Incomplete JSON should be detected
@@ -60,6 +63,7 @@ fn test_has_incomplete_tool_call_truncated_mid_value() {
         finished: false,
         tool_calls: None,
         usage: None,
+        stop_reason: None,
     };
     parser.process_chunk(&chunk);
     // Incomplete JSON should be detected
@@ -77,6 +81,7 @@ fn test_has_incomplete_tool_call_with_text_before() {
         finished: false,
         tool_calls: None,
         usage: None,
+        stop_reason: None,
     };
     parser.process_chunk(&chunk);
     // Incomplete JSON should be detected
@@ -93,6 +98,7 @@ fn test_has_incomplete_tool_call_malformed_like_trace() {
         finished: false,
         tool_calls: None,
         usage: None,
+        stop_reason: None,
     };
     parser.process_chunk(&chunk);
     // Truncated JSON (missing closing braces) should be detected as incomplete
@@ -113,6 +119,7 @@ fn test_has_unexecuted_tool_call_no_tool_pattern() {
         finished: false,
         tool_calls: None,
         usage: None,
+        stop_reason: None,
     };
     parser.process_chunk(&chunk);
     assert!(!parser.has_unexecuted_tool_call());
@@ -126,6 +133,7 @@ fn test_has_unexecuted_tool_call_complete_tool_call() {
         finished: false,
         tool_calls: None,
         usage: None,
+        stop_reason: None,
     };
     parser.process_chunk(&chunk);
     // Complete JSON tool call that wasn't executed should be detected
@@ -140,6 +148,7 @@ fn test_has_unexecuted_tool_call_incomplete_json() {
         finished: false,
         tool_calls: None,
         usage: None,
+        stop_reason: None,
     };
     parser.process_chunk(&chunk);
     // Incomplete JSON should NOT be detected as unexecuted (it's incomplete, not unexecuted)
@@ -157,6 +166,7 @@ Some trailing text after the JSON"#.to_string(),
         finished: false,
         tool_calls: None,
         usage: None,
+        stop_reason: None,
     };
     parser.process_chunk(&chunk);
     // Complete JSON tool call should be detected even with trailing text
@@ -175,6 +185,7 @@ I'll execute this command now."#.to_string(),
         finished: false,
         tool_calls: None,
         usage: None,
+        stop_reason: None,
     };
     parser.process_chunk(&chunk);
     // Complete JSON tool call should be detected

crates/g3-core/tests/streaming_parser_test.rs

@@ -17,6 +17,7 @@ fn chunk(content: &str, finished: bool) -> CompletionChunk {
         finished,
         tool_calls: None,
         usage: None,
+        stop_reason: None,
     }
 }