fix: add Tag.random() helper required by backported #23088 tests (#23094)

## Summary

CI3 on PR #23080 failed because the backported tests in
`log_service.test.ts` (from #23088) call `Tag.random()`, but the `Tag`
class on `v4-next` does not yet have that static factory — it was added
on `next` by #22516, which has not been backported.

This adds `Tag.random()` (and switches `Fr` from a type-only import) on
the v4-next branch. Mirrors the upstream definition exactly.

## Test plan

- CI on PR #23080 should now pass `pxe` unit tests once this lands on
`backport-to-v4-next-staging`.

ClaudeBox log: https://claudebox.work/s/3be07c7bc11fd1d8?run=1

Author: vezenovm

yarn-project/stdlib/src/logs/tag.ts

@@ -1,5 +1,5 @@
 import { poseidon2Hash } from '@aztec/foundation/crypto/poseidon';
-import type { Fr } from '@aztec/foundation/curves/bn254';
+import { Fr } from '@aztec/foundation/curves/bn254';
 import type { ZodFor } from '@aztec/foundation/schemas';
 
 import { schemas } from '../schemas/schemas.js';
@@ -36,6 +36,10 @@ export class Tag {
     return this.value.equals(other.value);
   }
 
+  static random(): Tag {
+    return new Tag(Fr.random());
+  }
+
   static get schema(): ZodFor<Tag> {
     return schemas.Fr.transform((fr: Fr) => new Tag(fr));
   }