S3 API: conditional read and write (#7154)

* conditional put

* more tests

* check all conditions

* address comments

* conditional multipart complete

* conditional reads

Read Operations (GET, HEAD):
If-None-Match / If-Modified-Since failures → 304 Not Modified ✅
If-Match / If-Unmodified-Since failures → 412 Precondition Failed ✅
Write Operations (PUT, CompleteMultipartUpload):
All conditional failures → 412 Precondition Failed ✅
Copy Operations (CopyObject):
Copy-source conditionals → 412 Precondition Failed (already implemented) ✅

* test actual code

* Interface-Based Testing

* cleanup

* Testing Interface

* Update s3api_object_handlers_put.go

* refactor

weed/s3api/s3api_object_handlers_multipart.go

@@ -114,6 +114,14 @@ func (s3a *S3ApiServer) CompleteMultipartUploadHandler(w http.ResponseWriter, r
 		return
 	}
 
+	// Check conditional headers before completing multipart upload
+	// This implements AWS S3 behavior where conditional headers apply to CompleteMultipartUpload
+	if errCode := s3a.checkConditionalHeaders(r, bucket, object); errCode != s3err.ErrNone {
+		glog.V(3).Infof("CompleteMultipartUploadHandler: Conditional header check failed for %s/%s", bucket, object)
+		s3err.WriteErrorResponse(w, r, errCode)
+		return
+	}
+
 	response, errCode := s3a.completeMultipartUpload(r, &s3.CompleteMultipartUploadInput{
 		Bucket:   aws.String(bucket),
 		Key:      objectKey(aws.String(object)),