s3: add s3:ExistingObjectTag condition support for bucket policies (#7677)

* s3: add s3:ExistingObjectTag condition support in policy engine

Add support for s3:ExistingObjectTag/<tag-key> condition keys in bucket
policies, allowing access control based on object tags.

Changes:
- Add ObjectEntry field to PolicyEvaluationArgs (entry.Extended metadata)
- Update EvaluateConditions to handle s3:ExistingObjectTag/<key> format
- Extract tag value from entry metadata using X-Amz-Tagging-<key> prefix

This enables policies like:
{
  "Condition": {
    "StringEquals": {
      "s3:ExistingObjectTag/status": ["public"]
    }
  }
}

Fixes: https://github.com/seaweedfs/seaweedfs/issues/7447

* s3: update EvaluatePolicy to accept object entry for tag conditions

Update BucketPolicyEngine.EvaluatePolicy to accept objectEntry parameter
(entry.Extended metadata) for evaluating tag-based policy conditions.

Changes:
- Add objectEntry parameter to EvaluatePolicy method
- Update callers in auth_credentials.go and s3api_bucket_handlers.go
- Pass nil for objectEntry in auth layer (entry fetched later in handlers)

For tag-based conditions to work, handlers should call EvaluatePolicy
with the object's entry.Extended after fetching the entry from filer.

* s3: add tests for s3:ExistingObjectTag policy conditions

Add comprehensive tests for object tag-based policy conditions:

- TestExistingObjectTagCondition: Basic tag matching scenarios
  - Matching/non-matching tag values
  - Missing tags, no tags, empty tags
  - Multiple tags with one matching

- TestExistingObjectTagConditionMultipleTags: Multiple tag conditions
  - Both tags match
  - Only one tag matches

- TestExistingObjectTagDenyPolicy: Deny policies with tag conditions
  - Default allow without tag
  - Deny when specific tag present

* s3: document s3:ExistingObjectTag support and feature status

Update policy engine documentation:

- Add s3:ExistingObjectTag/<tag-key> to supported condition keys
- Add 'Object Tag-Based Access Control' section with examples
- Add 'Feature Status' section with implemented and planned features

Planned features for future implementation:
- s3:RequestObjectTag/<key>
- s3:RequestObjectTagKeys
- s3:x-amz-server-side-encryption
- Cross-account access

* Implement tag-based policy re-check in handlers

- Add checkPolicyWithEntry helper to S3ApiServer for handlers to re-check
  policy after fetching object entry (for s3:ExistingObjectTag conditions)
- Add HasPolicyForBucket method to policy engine for efficient check
- Integrate policy re-check in GetObjectHandler after entry is fetched
- Integrate policy re-check in HeadObjectHandler after entry is fetched
- Update auth_credentials.go comments to explain two-phase evaluation
- Update documentation with supported operations for tag-based conditions

This implements 'Approach 1' where handlers re-check the policy with
the object entry after fetching it, allowing tag-based conditions to
be properly evaluated.

* Add integration tests for s3:ExistingObjectTag conditions

- Add TestCheckPolicyWithEntry: tests checkPolicyWithEntry helper with various
  tag scenarios (matching tags, non-matching tags, empty entry, nil entry)
- Add TestCheckPolicyWithEntryNoPolicyForBucket: tests early return when no policy
- Add TestCheckPolicyWithEntryNilPolicyEngine: tests nil engine handling
- Add TestCheckPolicyWithEntryDenyPolicy: tests deny policies with tag conditions
- Add TestHasPolicyForBucket: tests HasPolicyForBucket method

These tests cover the Phase 2 policy evaluation with object entry metadata,
ensuring tag-based conditions are properly evaluated.

* Address code review nitpicks

- Remove unused extractObjectTags placeholder function (engine.go)
- Add clarifying comment about s3:ExistingObjectTag/<key> evaluation
- Consolidate duplicate tag-based examples in README
- Factor out tagsToEntry helper to package level in tests

* Address code review feedback

- Fix unsafe type assertions in GetObjectHandler and HeadObjectHandler
  when getting identity from context (properly handle type assertion failure)
- Extract getConditionContextValue helper to eliminate duplicated logic
  between EvaluateConditions and EvaluateConditionsLegacy
- Ensure consistent handling of missing condition keys (always return
  empty slice)

* Fix GetObjectHandler to match HeadObjectHandler pattern

Add safety check for nil objectEntryForSSE before tag-based policy
evaluation, ensuring tag-based conditions are always evaluated rather
than silently skipped if entry is unexpectedly nil.

Addresses review comment from Copilot.

* Fix HeadObject action name in docs for consistency

Change 'HeadObject' to 's3:HeadObject' to match other action names.

* Extract recheckPolicyWithObjectEntry helper to reduce duplication

Move the repeated identity extraction and policy re-check logic from
GetObjectHandler and HeadObjectHandler into a shared helper method.

* Add validation for empty tag key in s3:ExistingObjectTag condition

Prevent potential issues with malformed policies containing
s3:ExistingObjectTag/ (empty tag key after slash).

weed/s3api/s3api_server.go

@@ -252,6 +252,62 @@ func (s3a *S3ApiServer) syncBucketPolicyToEngine(bucket string, policyDoc *polic
 	}
 }
 
+// checkPolicyWithEntry re-evaluates bucket policy with the object entry metadata.
+// This is used by handlers after fetching the entry to enforce tag-based conditions
+// like s3:ExistingObjectTag/<key>.
+//
+// Returns:
+//   - s3err.ErrCode: ErrNone if allowed, ErrAccessDenied if denied
+//   - bool: true if policy was evaluated (has policy for bucket), false if no policy
+func (s3a *S3ApiServer) checkPolicyWithEntry(r *http.Request, bucket, object, action, principal string, objectEntry map[string][]byte) (s3err.ErrorCode, bool) {
+	if s3a.policyEngine == nil {
+		return s3err.ErrNone, false
+	}
+
+	// Skip if no policy for this bucket
+	if !s3a.policyEngine.HasPolicyForBucket(bucket) {
+		return s3err.ErrNone, false
+	}
+
+	allowed, evaluated, err := s3a.policyEngine.EvaluatePolicy(bucket, object, action, principal, r, objectEntry)
+	if err != nil {
+		glog.Errorf("checkPolicyWithEntry: error evaluating policy for %s/%s: %v", bucket, object, err)
+		return s3err.ErrInternalError, true
+	}
+
+	if !evaluated {
+		return s3err.ErrNone, false
+	}
+
+	if !allowed {
+		glog.V(3).Infof("checkPolicyWithEntry: policy denied access to %s/%s for principal %s", bucket, object, principal)
+		return s3err.ErrAccessDenied, true
+	}
+
+	return s3err.ErrNone, true
+}
+
+// recheckPolicyWithObjectEntry performs the second phase of policy evaluation after
+// an object's entry is fetched. It extracts identity from context and checks for
+// tag-based conditions like s3:ExistingObjectTag/<key>.
+//
+// Returns s3err.ErrNone if allowed, or an error code if denied or on error.
+func (s3a *S3ApiServer) recheckPolicyWithObjectEntry(r *http.Request, bucket, object, action string, objectEntry map[string][]byte, handlerName string) s3err.ErrorCode {
+	identityRaw := GetIdentityFromContext(r)
+	var identity *Identity
+	if identityRaw != nil {
+		var ok bool
+		identity, ok = identityRaw.(*Identity)
+		if !ok {
+			glog.Errorf("%s: unexpected identity type in context for %s/%s", handlerName, bucket, object)
+			return s3err.ErrInternalError
+		}
+	}
+	principal := buildPrincipalARN(identity)
+	errCode, _ := s3a.checkPolicyWithEntry(r, bucket, object, action, principal, objectEntry)
+	return errCode
+}
+
 // classifyDomainNames classifies domains into path-style and virtual-host style domains.
 // A domain is considered path-style if:
 //  1. It contains a dot (has subdomains)