fix(coding-agent): coerce stringified JSON edits in edit tool (#3370)

Some models (Opus 4.6, GLM-5.1) send the edits parameter as a JSON
string instead of a parsed array. This fails AJV validation with
'must be array' and models fall back to sed/python.

Parse stringified edits in prepareEditArguments before validation.

packages/coding-agent/test/edit-tool-legacy-input.test.ts

@@ -88,3 +88,29 @@ describe("edit tool prepareArguments", () => {
 		expect(await readFile(filePath, "utf8")).toBe("after\n");
 	});
 });
+
+describe("edit tool stringified edits", () => {
+	it("parses edits from a JSON string", () => {
+		const definition = createEditToolDefinition(process.cwd());
+		const prepared = definition.prepareArguments!({
+			path: "file.txt",
+			edits: JSON.stringify([{ oldText: "a", newText: "b" }]),
+		});
+		expect(prepared).toEqual({
+			path: "file.txt",
+			edits: [{ oldText: "a", newText: "b" }],
+		});
+	});
+
+	it("leaves edits alone when the string is not valid JSON", () => {
+		const definition = createEditToolDefinition(process.cwd());
+		const prepared = definition.prepareArguments!({
+			path: "file.txt",
+			edits: "not json",
+		});
+		expect(prepared).toEqual({
+			path: "file.txt",
+			edits: "not json",
+		});
+	});
+});