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/s3err/s3api_errors.go

@@ -102,6 +102,7 @@ const (
 	ErrAuthNotSetup
 	ErrNotImplemented
 	ErrPreconditionFailed
+	ErrNotModified
 
 	ErrExistingObjectIsDirectory
 	ErrExistingObjectIsFile
@@ -451,6 +452,11 @@ var errorCodeResponse = map[ErrorCode]APIError{
 		Description:    "At least one of the pre-conditions you specified did not hold",
 		HTTPStatusCode: http.StatusPreconditionFailed,
 	},
+	ErrNotModified: {
+		Code:           "NotModified",
+		Description:    "The object was not modified since the specified time",
+		HTTPStatusCode: http.StatusNotModified,
+	},
 	ErrExistingObjectIsDirectory: {
 		Code:           "ExistingObjectIsDirectory",
 		Description:    "Existing Object is a directory.",