Support streaming the new file content for edit file tool calls

Currently edit_file tool shows only metadata during streaming (target_file, is_new_file)

• File content should be streamed progressively as it's being generated
• Update EditFileTool component to display partial content during streaming state
• Enhance the streaming architecture to include file content in partial args

Start with the EditFileTool component and trace the streaming architecture that supplies partial args. Confirm how target_file and is_new_file are currently exposed, then verify that file content appears progressively during streaming and remains available in the completed tool call.

Written by the indexing model from the issue text.