feat(coding-agent/edit): added hashline fallback preview for in-flight edits

- Added edit streaming fallback data to the edit render context and used it when no finalized patch text was available.
- Implemented hashline fallback rendering that strips envelope syntax, sanitizes text, and truncates output to a fixed number of lines with a streaming indicator.
- Updated tool execution to populate the fallback preview from the active edit strategy and added a renderer test for hashline envelope input while the diff is not yet computable.

packages/coding-agent/src/modes/components/tool-execution.ts

@@ -665,6 +665,12 @@ export class ToolExecutionComponent extends Container {
 					context.perFileDiffPreview = previews;
 				}
 			}
+			if (!previews?.some(preview => preview.diff)) {
+				const editMode = this.#editMode;
+				const strategy = editMode ? EDIT_MODE_STRATEGIES[editMode] : undefined;
+				const fallback = strategy?.renderStreamingFallback(this.#args, theme);
+				if (fallback) context.editStreamingFallback = fallback;
+			}
 			context.renderDiff = renderDiff;
 		}