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/policy_engine/engine.go

@@ -91,6 +91,14 @@ func (engine *PolicyEngine) DeleteBucketPolicy(bucketName string) error {
 	return nil
 }
 
+// HasPolicyForBucket checks if a bucket has a policy configured
+func (engine *PolicyEngine) HasPolicyForBucket(bucketName string) bool {
+	engine.mutex.RLock()
+	defer engine.mutex.RUnlock()
+	_, exists := engine.contexts[bucketName]
+	return exists
+}
+
 // EvaluatePolicy evaluates a policy for the given arguments
 func (engine *PolicyEngine) EvaluatePolicy(bucketName string, args *PolicyEvaluationArgs) PolicyEvaluationResult {
 	engine.mutex.RLock()
@@ -154,7 +162,7 @@ func (engine *PolicyEngine) evaluateStatement(stmt *CompiledStatement, args *Pol
 
 	// Check conditions
 	if len(stmt.Statement.Condition) > 0 {
-		if !EvaluateConditions(stmt.Statement.Condition, args.Conditions) {
+		if !EvaluateConditions(stmt.Statement.Condition, args.Conditions, args.ObjectEntry) {
 			return false
 		}
 	}
@@ -201,10 +209,8 @@ func ExtractConditionValuesFromRequest(r *http.Request) map[string][]string {
 		values["aws:Referer"] = []string{referer}
 	}
 
-	// S3 object-level conditions
-	if r.Method == "GET" || r.Method == "HEAD" {
-		values["s3:ExistingObjectTag"] = extractObjectTags(r)
-	}
+	// Note: s3:ExistingObjectTag/<key> conditions are evaluated using objectEntry
+	// passed to EvaluatePolicy, not extracted from the request.
 
 	// S3 bucket-level conditions
 	if delimiter := r.URL.Query().Get("delimiter"); delimiter != "" {
@@ -243,13 +249,6 @@ func ExtractConditionValuesFromRequest(r *http.Request) map[string][]string {
 	return values
 }
 
-// extractObjectTags extracts object tags from request (placeholder implementation)
-func extractObjectTags(r *http.Request) []string {
-	// This would need to be implemented based on how object tags are stored
-	// For now, return empty slice
-	return []string{}
-}
-
 // BuildResourceArn builds an ARN for the given bucket and object
 func BuildResourceArn(bucketName, objectName string) string {
 	if objectName == "" {
@@ -352,15 +351,6 @@ func GetObjectNameFromArn(arn string) string {
 	return ""
 }
 
-// HasPolicyForBucket checks if a bucket has a policy
-func (engine *PolicyEngine) HasPolicyForBucket(bucketName string) bool {
-	engine.mutex.RLock()
-	defer engine.mutex.RUnlock()
-
-	_, exists := engine.contexts[bucketName]
-	return exists
-}
-
 // GetPolicyStatements returns all policy statements for a bucket
 func (engine *PolicyEngine) GetPolicyStatements(bucketName string) []PolicyStatement {
 	engine.mutex.RLock()