Keep deprecated Router.route() a nullable probe

Callers using the deprecated `Router` from
`@fedify/fedify/federation` could pass arbitrary strings, including
absolute URLs or otherwise non-path inputs, and get `null` back when
they did not match.  After the switch to `@fedify/uri-template`, the
`assertPath(url)` call made the same probing code throw `RouterError`
instead.

Since this class is still the deprecated compatibility surface and
this change ships in a minor release, that is a compatibility
regression rather than a migration nudge.  Replace `assertPath(url)`
with an `isPath(url)` guard so `route()` again returns `null` for
non-path inputs, while the underlying `@fedify/uri-template` router
stays strict.  `add()` is left throwing because the old
`Router.add()` also threw on invalid templates.

#758 (comment)
#418

Assisted-by: Claude Code:claude-opus-4-7

Author: 2chanhaeng

packages/fedify/src/federation/router.test.ts

@@ -108,6 +108,17 @@ test("Router.build()", () => {
   );
 });
 
+test("Router.route() returns null for non-path inputs", () => {
+  const router = setUp();
+  // The old Fedify 2.x `Router` returned `null` (not threw) when probed
+  // with non-path inputs such as absolute URLs.
+  assertEquals(router.route("https://example.com/users/alice"), null);
+  assertEquals(router.route("users/alice"), null);
+  assertEquals(router.route("not a path"), null);
+  // Valid paths that simply do not match still return null.
+  assertEquals(router.route("/unknown"), null);
+});
+
 test("Compatibility between RouterErrors", () => {
   const newError = new UriTemplateRouterError("boom");
   assert(newError instanceof UriTemplateRouterError);

packages/fedify/src/federation/router.ts

@@ -4,6 +4,7 @@ import type {
 } from "@fedify/uri-template";
 import {
   assertPath,
+  isPath,
   Router as _Router,
   RouterError as _RouterError,
 } from "@fedify/uri-template";
@@ -112,11 +113,14 @@ export class Router {
    * @param url The URL to resolve.
    * @returns The name of the path and its values, if any match.  Otherwise,
    *          `null`.
-   * @deprecated Use `Router` from `@fedify/uri-template` instead.
+   * @deprecated Use `Router` from `@fedify/uri-template` instead.  Unlike the
+   *             stricter `@fedify/uri-template` router, this compatibility
+   *             method keeps the old Fedify 2.x contract of returning `null`
+   *             (rather than throwing) for inputs that are not router paths.
    */
   route(url: string): RouterRouteResult | null {
     return convertRouterError(() => {
-      assertPath(url);
+      if (!isPath(url)) return null;
       return this.#router.route(url);
     });
   }