Add tests for duplicate detection logic

Added 13 tests to verify that duplicate detection only catches
IMMEDIATELY SEQUENTIAL duplicates:

- test_find_complete_json_object_end_* - Tests for JSON parsing helper
- test_same_tool_with_text_between_not_duplicate - Key test ensuring
  tool calls separated by text are NOT duplicates
- test_different_tools_back_to_back_not_duplicate
- test_same_tool_different_args_not_duplicate
- test_identical_tool_calls_back_to_back_are_duplicates
- test_has_text_after_tool_call - Tests text detection logic
- test_tool_call_with_newlines_between
- test_tool_call_with_whitespace_text_between
- test_tool_call_in_middle_of_text
- test_multiple_different_tool_calls_with_text

Also made find_complete_json_object_end public for testing.

crates/g3-core/src/lib.rs

@@ -534,7 +534,8 @@ impl StreamingToolParser {
 
     /// Find the end position (byte index) of a complete JSON object in the text
     /// Returns None if no complete JSON object is found
-    fn find_complete_json_object_end(text: &str) -> Option<usize> {
+    /// Find the end position (byte index) of a complete JSON object in the text
+    pub fn find_complete_json_object_end(text: &str) -> Option<usize> {
         let mut brace_count = 0;
         let mut in_string = false;
         let mut escape_next = false;