[Bug] Proxy ACL metadata providers return null futures for unsupported mutations

### Before Creating the Bug Report

- [x] I found a bug, not just asking a question, which should be created in GitHub Issues.
- [x] I have searched the existing issues and believe this is not a duplicate.
- [x] I have confirmed that this bug belongs to the current repository.

### Describe the Bug

`ProxyAuthenticationMetadataProvider` and `ProxyAuthorizationMetadataProvider` implement async metadata provider interfaces, but several unsupported methods currently return `null` instead of a `CompletableFuture`.

Examples:

- `ProxyAuthenticationMetadataProvider#createUser`
- `ProxyAuthenticationMetadataProvider#deleteUser`
- `ProxyAuthenticationMetadataProvider#updateUser`
- `ProxyAuthenticationMetadataProvider#listUser`
- `ProxyAuthorizationMetadataProvider#createAcl`
- `ProxyAuthorizationMetadataProvider#deleteAcl`
- `ProxyAuthorizationMetadataProvider#updateAcl`
- `ProxyAuthorizationMetadataProvider#listAcl`

Returning `null` from an async provider is unsafe because callers naturally treat these methods as returning a `CompletableFuture`. That can turn an unsupported operation into a `NullPointerException`, hiding the real capability gap and making Proxy ACL diagnostics harder.

### Expected Behavior

Unsupported mutation/list operations should never return `null`. They should either:

1. delegate to the underlying metadata service if the operation is supported; or
2. return a failed `CompletableFuture` with a clear `UnsupportedOperationException` / diagnostic message.

The existing read paths (`getUser`, `getAcl`) should keep their current behavior.

### Scope

RocketMQ Studio Track 2 / Proxy Admin authentication and authorization foundation. This also improves the AUTH-01 call-chain behavior expected by Control Plane integrations.

### Proposed Fix

Return failed futures for currently unsupported operations, with method-specific messages. Add unit tests to assert every unsupported method returns a non-null future that completes exceptionally instead of returning `null`.

Start by locating ProxyAuthenticationMetadataProvider and ProxyAuthorizationMetadataProvider and read the implementations of the listed unsupported methods, then find their unit tests or add tests alongside them. Verify each method returns a non-null CompletableFuture that completes exceptionally with a method-specific unsupported-operation diagnostic, while getUser and getAcl retain their current behavior.

Written by the indexing model from the issue text.