* s3: register account-less identities' synthesized account in the lookup
#9962 gave each account-less identity a distinct account id derived from
its name (instead of collapsing into admin), but never registered that
account in the id->account map. GetAccountNameById then returned empty
for such ids, so ACL grantee validation rejected canonical grants to the
caller's own account with InvalidRequest, and bucket/object owner display
was dropped as 'owner is invalid'.
This broke a canned PutObjectAcl by an account-less identity (e.g.
TestVersionedObjectAcl with the default 'some_admin_user' identity):
ValidateAndTransferGrants -> GetAccountNameById -> 'account id is not
exists' -> 400 InvalidRequest.
Register the synthesized account at config load so its id resolves to a
display name. Add a regression test.
* s3: reuse explicitly-configured account for account-less identity
Address review: if an account with the same id as an account-less
identity's synthesized account is explicitly configured (custom display
name/email), reuse it instead of the synthesized one. Add a test.
* s3: stop collapsing account-less identities into the admin account
Identities configured without an account block all defaulted to the
shared admin account, so distinct users got the same owner id and
ownership checks could not tell them apart. checkAccessByOwnership also
treated that id as an admin bypass, so any account-less caller passed
ownership for any bucket. Give such identities a distinct account id from
their name, and decide the ownership admin bypass by Admin capability
rather than by the account id. isUserAdmin is now nil-safe.
* s3: use the context identity in isUserAdmin before re-authenticating
The Auth middleware already verifies and stores the identity in the
request context. Read it there first so the ownership/admin checks don't
re-run signature verification, which is redundant and fails once the
request body has been consumed.
* s3: nil-guard the context identity in isUserAdmin
A non-nil interface wrapping a typed-nil *Identity passes the type
assertion; guard against it before calling isAdmin().
* s3: trim verbose comments
Chris Lu