Re-enable guardian E2E on web-sdk 0.16 once OZ ships a protocol-0.16 multisig/guardian client

## Context

The `next` branch is migrating to web-sdk 0.16 (PR #521). Guardian account **creation** fails on SDK 0.16 with a MASM link error:

```
Error creating Guardian account: script builder: failed to link module
openzeppelin::auth::guardian_ecdsa … undefined constant 'AUTH_UNAUTHORIZED_EVENT'
```

### Root cause
`@openzeppelin/miden-multisig-client` has **no release built against miden-protocol 0.16**. The latest (`0.16.1`) still depends on `@miden-sdk/miden-sdk@^0.15.8`; its `guardian_ecdsa` MASM references `AUTH_UNAUTHORIZED_EVENT`, a 0.15-era auth constant removed/renamed in protocol 0.16.0-alpha.4. OZ's `0.16.x` version tracks their own client, not miden-protocol 0.16.

This breaks guardian in the **product** on 0.16, not only in E2E. A client-only MASM patch is not viable (co-sign ABI must match across client + guardian server + on-chain verification).

### Quarantined on #521 (re-enable when OZ ships a protocol-0.16 client)
- [ ] `pr-e2e-bridge-guardian.yml` — remove the job-level `if: false` guard
- [ ] `pr-e2e-guardian-lifecycle.yml` — remove the job-level `if: false` guard
- [ ] `playwright.swap.config.ts` — remove the `testIgnore` for `swap-guardian.spec.ts`
- [ ] Revert both guardian gates to require `success` only (drop the `skipped` tolerance)
- [ ] Bump `@openzeppelin/miden-multisig-client` + `@openzeppelin/guardian-client` to the protocol-0.16 release; bump `GUARDIAN_IMAGE_TAG` to the matching server image

Non-guardian E2E (local, earn, swap non-guardian) is green on 0.16.