fix(s3): omit NotResource:null from bucket policy JSON response (#8658)

* fix(s3): omit NotResource:null from bucket policy JSON response (#8657)

Change NotResource from value type to pointer (*StringOrStringSlice) so
that omitempty properly omits it when unset, matching the existing
Principal field pattern. This prevents IaC tools (Terraform, Ansible)
from detecting false configuration drift.

Add bucket policy round-trip idempotency integration tests.

* simplify JSON comparison in bucket policy idempotency test

Use require.JSONEq directly on the raw JSON strings instead of
round-tripping through unmarshal/marshal, since JSONEq already
handles normalization internally.

* fix bucket policy test cases that locked out the admin user

The Deny+NotResource test cases used Action:"s3:*" which denied the
admin's own GetBucketPolicy call. Scope deny to s3:GetObject only,
and add an Allow+NotResource variant instead.

* fix(s3): also make Resource a pointer to fix empty string in JSON

Apply the same omitempty pointer fix to the Resource field, which was
emitting "Resource":"" when only NotResource was set. Add
NewStringOrStringSlicePtr helper, make Strings() nil-safe, and handle
*StringOrStringSlice in normalizeToStringSliceWithError.

* improve bucket policy integration tests per review feedback

- Replace time.Sleep with waitForClusterReady using ListBuckets
- Use structural hasKey check instead of brittle substring NotContains
- Assert specific NoSuchBucketPolicy error code after delete
- Handle single-statement policies in hasKey helper

weed/s3api/s3api_embedded_iam_test.go

@@ -504,7 +504,7 @@ func TestEmbeddedIamAttachUserPolicyRefreshesIAM(t *testing.T) {
 			{
 				Effect:   policy_engine.PolicyEffectAllow,
 				Action:   policy_engine.NewStringOrStringSlice("s3:GetObject"),
-				Resource: policy_engine.NewStringOrStringSlice("arn:aws:s3:::bucket/*"),
+				Resource: policy_engine.NewStringOrStringSlicePtr("arn:aws:s3:::bucket/*"),
 			},
 		},
 	}