fix(replication): resume partial chunk reads on EOF instead of re-downloading (#8607)

* fix(replication): resume partial chunk reads on EOF instead of re-downloading

When replicating chunks and the source connection drops mid-transfer,
accumulate the bytes already received and retry with a Range header
to fetch only the remaining bytes. This avoids re-downloading
potentially large chunks from scratch on each retry, reducing load
on busy source servers and speeding up recovery.

* test(replication): add tests for downloadWithRange including gzip partial reads

Tests cover:
- No offset (no Range header sent)
- With offset (Range header verified)
- Content-Disposition filename extraction
- Partial read + resume: server drops connection mid-transfer, client
  resumes with Range from the offset of received bytes
- Gzip partial read + resume: first response is gzip-encoded (Go auto-
  decompresses), connection drops, resume request gets decompressed data
  (Go doesn't add Accept-Encoding when Range is set, so the server
  decompresses), combined bytes match original

* fix(replication): address PR review comments

- Consolidate downloadWithRange into DownloadFile with optional offset
  parameter (variadic), eliminating code duplication (DRY)
- Validate HTTP response status: require 206 + correct Content-Range
  when offset > 0, reject when server ignores Range header
- Use if/else for fullData assignment for clarity
- Add test for rejected Range (server returns 200 instead of 206)

* refactor(replication): remove unused ReplicationSource interface

The interface was never referenced and its signature didn't match
the actual FilerSource.ReadPart method.

---------

Co-authored-by: Copilot <copilot@github.com>

weed/util/http/http_global_client_util.go

@@ -201,7 +201,7 @@ func GetUrlStream(url string, values url.Values, readFn func(io.Reader) error) e
 	return readFn(r.Body)
 }
 
-func DownloadFile(fileUrl string, jwt string) (filename string, header http.Header, resp *http.Response, e error) {
+func DownloadFile(fileUrl string, jwt string, offset ...int64) (filename string, header http.Header, resp *http.Response, e error) {
 	req, err := http.NewRequest(http.MethodGet, fileUrl, nil)
 	if err != nil {
 		return "", nil, nil, err
@@ -209,10 +209,29 @@ func DownloadFile(fileUrl string, jwt string) (filename string, header http.Head
 
 	maybeAddAuth(req, jwt)
 
+	var rangeOffset int64
+	if len(offset) > 0 {
+		rangeOffset = offset[0]
+	}
+	if rangeOffset > 0 {
+		req.Header.Set("Range", fmt.Sprintf("bytes=%d-", rangeOffset))
+	}
+
 	response, err := GetGlobalHttpClient().Do(req)
 	if err != nil {
 		return "", nil, nil, err
 	}
+
+	if rangeOffset > 0 {
+		expected := fmt.Sprintf("bytes %d-", rangeOffset)
+		if response.StatusCode != http.StatusPartialContent ||
+			!strings.HasPrefix(response.Header.Get("Content-Range"), expected) {
+			CloseResponse(response)
+			return "", nil, nil, fmt.Errorf("range request %q to %s returned %s with Content-Range %q",
+				req.Header.Get("Range"), fileUrl, response.Status, response.Header.Get("Content-Range"))
+		}
+	}
+
 	header = response.Header
 	contentDisposition := response.Header["Content-Disposition"]
 	if len(contentDisposition) > 0 {