test(admin-auth): unit + e2e coverage for the admin authorization gate
Adds the first tests for the central admin authorization gate
`rustfs/src/admin/auth.rs`, which previously had zero coverage
(backlog#1151 sec-4, master plan #1155).
Unit tests (rustfs/src/admin/auth.rs):
- Refactor the two `validate_admin_request*` entry points to share a
single generic decision core `evaluate_admin_actions<S: Store>` /
`check_admin_request_auth<S: Store>` (removes the duplicated
action-loop and makes the gate testable without a running cluster).
- Cover: owner/root credential allowed; authenticated non-admin
credential denied with AccessDenied; missing credential denied; and
the multi-action loop grants on any permitted action, denies when
none pass. Backed by an in-memory empty IamSys so the owner path
short-circuits to allow and every other principal resolves to deny.
E2E (crates/e2e_test/src/admin_auth_test.rs, raw SigV4 over HTTP, no
awscurl dependency):
- non_admin_credential_denied_on_admin_api: a limited IAM user gets
403 AccessDenied on GET /rustfs/admin/v3/info while root succeeds.
- root_credential_rotation_takes_effect: restart with rotated
--access-key/--secret-key; new credential works and the stale one is
rejected, on both the admin plane and the S3 data plane.
- default_credentials_emit_startup_warning: booting with the default
rustfsadmin credentials emits the default-credentials warning.
Refs rustfs/backlog#1151 (sec-4), #1155.
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