Tighten relay fixtures and actor assertions

Move Bob's store-only inbox onto a distinct intercepted IPv6 address to
keep the fixture convention consistent without sharing the destination
recorded by the forwarding test.

Require the embedded actor's id in both Accept delivery helpers. Removing
the URL-string fallback makes serialization-shape regressions fail the
assertion instead of accepting either representation.

Addresses these review comments:

#1047 (comment)
#1047 (comment)

Validated formatting and all 58 relay Deno test steps. Node.js and Bun
were not rerun for these edits; the preceding fresh-worktree attempt was
blocked by dependency-download timeouts at npm.jsr.io.

Codex applied both requested review fixes and ran automated checks.
Human manual verification was not recorded.

Changelog: none
Assisted-by: Codex:gpt-6

Author: sij411

packages/relay/src/litepub.test.ts

@@ -156,7 +156,7 @@ function assertAcceptDelivered(
   strictEqual(accepts[0].body.actor, "https://relay.example.com/users/relay");
   strictEqual(accepts[0].body.object.type, "Follow");
   strictEqual(
-    accepts[0].body.object.actor?.id ?? accepts[0].body.object.actor,
+    accepts[0].body.object.actor.id,
     follow.actorId?.href,
   );
 }

packages/relay/src/mastodon.test.ts

@@ -122,7 +122,7 @@ function assertAcceptDelivered(
   strictEqual(accepts[0].body.actor, "https://relay.example.com/users/relay");
   strictEqual(accepts[0].body.object.type, "Follow");
   strictEqual(
-    accepts[0].body.object.actor?.id ?? accepts[0].body.object.actor,
+    accepts[0].body.object.actor.id,
     follow.actorId?.href,
   );
 }
@@ -1047,7 +1047,7 @@ describe("MastodonRelay", () => {
     const follower2 = new Person({
       id: new URL("https://remote.example.com/users/bob"),
       preferredUsername: "bob",
-      inbox: new URL("https://remote.example.com/users/bob/inbox"),
+      inbox: new URL(`${INBOX_PREFIX}3]/users/bob/inbox`),
     });
 
     await kv.set(