docs(coding-agent): clarify that tool errors must be thrown, not returned

Returning { isError: true } from a tool's execute function was silently
ignored - the agent loop only sets isError via the catch block. Fix the
with-deps example to throw instead, add a clear note in the Tool Definition
docs section, and update the Error Handling summary.

closes #1881

packages/coding-agent/examples/extensions/with-deps/index.ts

@@ -21,11 +21,7 @@ export default function (pi: ExtensionAPI) {
 		execute: async (_toolCallId, params) => {
 			const result = ms(params.duration as ms.StringValue);
 			if (result === undefined) {
-				return {
-					content: [{ type: "text", text: `Invalid duration: "${params.duration}"` }],
-					isError: true,
-					details: {},
-				};
+				throw new Error(`Invalid duration: "${params.duration}"`);
 			}
 			return {
 				content: [{ type: "text", text: `${params.duration} = ${result} milliseconds` }],