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/s3_constants/header.go

@@ -57,6 +57,12 @@ const (
 	AmzObjectLockRetainUntilDate = "X-Amz-Object-Lock-Retain-Until-Date"
 	AmzObjectLockLegalHold       = "X-Amz-Object-Lock-Legal-Hold"
 
+	// S3 conditional headers
+	IfMatch           = "If-Match"
+	IfNoneMatch       = "If-None-Match"
+	IfModifiedSince   = "If-Modified-Since"
+	IfUnmodifiedSince = "If-Unmodified-Since"
+
 	// S3 conditional copy headers
 	AmzCopySourceIfMatch           = "X-Amz-Copy-Source-If-Match"
 	AmzCopySourceIfNoneMatch       = "X-Amz-Copy-Source-If-None-Match"