sftp: keep a file's existing bytes when it is opened for write without truncate (#11723)

* sftp: keep a file's existing bytes when it is opened for write without truncate

* sftp: preserve sparse gaps and refuse unreadable entries when copying existing content

The copy used a single chunk-stream read, which ends at the first sparse
gap: the tail past a hole was dropped, and a remote-only entry with no
local chunks copied zero bytes, so closing the handle uploaded an empty
file. Read through PrepareStreamContent, which fills unstored regions
with zeros, and refuse entries that carry no local bytes at all.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* filer: report a failed destination write from a completed chunk fetch

retriedStreamFetchChunkData broke out of the retry loop on a clean HTTP
end before checking writeErr, so a destination write that failed mid-
stream returned a short write and nil error. Check writeErr first.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* sftp: cover remote and all-holes sizes when copying existing content

Take the declared size from RemoteEntry when it exceeds FileSize, pad
inline content to the declared size, and treat a chunkless local file as
all holes rather than unreadable. A remote-only entry still refuses the
copy, now checked with a zero FileSize as well.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* sftp: size the existing-content copy the way filer.FileSize does

A locally shortened file still names its older, larger remote object;
taking RemoteSize unconditionally padded the copy back out to it.
filer.FileSize only honors the remote size when the remote is newer,
and also covers the chunk-extent size. A remote-only entry still
refuses the copy whenever it points at remote bytes.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: Chris Lu <chrislusf@users.noreply.github.com>
Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

test/sftp/basic_test.go

@@ -3,6 +3,7 @@ package sftp
 import (
 	"bytes"
 	"io"
+	"os"
 	"path"
 	"testing"
 
@@ -602,6 +603,32 @@ func TestFileContent(t *testing.T) {
 		sftpClient.Remove("/binary.bin")
 	})
 
+	t.Run("WriteAtOffsetKeepsExistingBytes", func(t *testing.T) {
+		file, err := sftpClient.Create("/resume.txt")
+		require.NoError(t, err)
+		_, err = file.Write([]byte("hello world"))
+		require.NoError(t, err)
+		require.NoError(t, file.Close())
+
+		// Reopen without O_TRUNC and write past the start, as a resumed upload does
+		file, err = sftpClient.OpenFile("/resume.txt", os.O_WRONLY)
+		require.NoError(t, err)
+		_, err = file.WriteAt([]byte("WORLD!"), 6)
+		require.NoError(t, err)
+		require.NoError(t, file.Close())
+
+		readFile, err := sftpClient.Open("/resume.txt")
+		require.NoError(t, err)
+		content, err := io.ReadAll(readFile)
+		require.NoError(t, err)
+		readFile.Close()
+
+		require.Equal(t, "hello WORLD!", string(content))
+
+		// Clean up
+		sftpClient.Remove("/resume.txt")
+	})
+
 	t.Run("EmptyFile", func(t *testing.T) {
 		file, err := sftpClient.Create("/empty.txt")
 		require.NoError(t, err)