test(ilm): deny manual transition for non-admin

[houseme]

Related Issues

Related to rustfs/backlog#1481 and rustfs/backlog#1476.

Summary of Changes

• Add a real-server e2e regression test for the manual ILM transition admin endpoint.
• The test first proves the root credential reaches the enqueue_only handler contract, then verifies a limited authenticated user receives 403 AccessDenied on the same POST /rustfs/admin/v3/ilm/transition/run request.

Verification

• cargo fmt --all --check
• git diff --check
• cargo test -p e2e_test non_admin_credential_denied_on_manual_transition_run -- --nocapture

Impact

Test coverage only. No public API, wire format, console behavior, or compatibility change.

Additional Notes

The e2e test covers the backend permission boundary only; console operations remain out of scope.

Adversarial validation: correctness attacked root handler reachability and limited-user denial with no break found; simplicity found the test reuses the existing signed request and limited-user e2e helpers; security attacked the authenticated-but-unauthorized admin endpoint case and observed 403 AccessDenied; concurrency/durability found no production lock, IO, or persisted-state change and the e2e is serial like neighboring admin auth tests; compatibility found no endpoint or response contract change beyond assertions; performance found test-only cold build/runtime cost; test-coverage found the new e2e fails if the admin gate allows the limited user or if the root path no longer reaches the enqueue-only handler.

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