fix(core): Duration.toString() throws an error (#18243)

`Duration.toString()` was intended to produce a value that would throw an exception when resolved,
but unintentionally was written to always throw immediately (the reason it was throwing is that `Token.asString()`
doesn't accept functions, it only accepts data values--`Lazy.string()` should have been used).

Instead, we remove the validation completely. `toString()` now produces a meaningless string, and users
should avoid using the `Duration` object in a context where it will be implicitly converted to a string.

Fixes #18176.


----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/core/lib/duration.ts

@@ -220,17 +220,13 @@ export class Duration {
   }
 
   /**
-   * Returns a string representation of this `Duration` that is also a Token that cannot be successfully resolved. This
-   * protects users against inadvertently stringifying a `Duration` object, when they should have called one of the
-   * `to*` methods instead.
+   * Returns a string representation of this `Duration`
+   *
+   * This is is never the right function to use when you want to use the `Duration`
+   * object in a template. Use `toSeconds()`, `toMinutes()`, `toDays()`, etc. instead.
    */
   public toString(): string {
-    return Token.asString(
-      () => {
-        throw new Error('Duration.toString() was used, but .toSeconds, .toMinutes or .toDays should have been called instead');
-      },
-      { displayHint: `${this.amount} ${this.unit.label}` },
-    );
+    return `Duration.${this.unit.label}(${this.amount})`;
   }
 
   /**

packages/@aws-cdk/core/test/duration.test.ts

@@ -4,8 +4,10 @@ import { Duration, Lazy, Stack, Token } from '../lib';
 describe('duration', () => {
   test('negative amount', () => {
     expect(() => Duration.seconds(-1)).toThrow(/negative/);
+  });
 
-
+  test('can stringify', () => {
+    expect(`${Duration.hours(1)}`).toEqual('Duration.hours(1)');
   });
 
   test('unresolved amount', () => {