Reimplement fs.FileInfo interface (#13315)

Co-authored-by: R.B. Boyer <4903+rboyer@users.noreply.github.com>

agent/uiserver/buffered_file.go

@@ -1,20 +1,31 @@
 package uiserver
 
 import (
-	"io"
+	"bytes"
 	"io/fs"
+	"os"
+	"time"
 )
 
 // bufferedFile implements fs.File and allows us to modify a file from disk by
 // writing out the new version into a buffer and then serving file reads from
 // that.
 type bufferedFile struct {
-	buf  io.Reader
+	buf  *bytes.Buffer
 	info fs.FileInfo
 }
 
+var _ fs.FileInfo = (*bufferedFile)(nil)
+
+func newBufferedFile(buf *bytes.Buffer, info fs.FileInfo) *bufferedFile {
+	return &bufferedFile{
+		buf:  buf,
+		info: info,
+	}
+}
+
 func (b *bufferedFile) Stat() (fs.FileInfo, error) {
-	return b.info, nil
+	return b, nil
 }
 
 func (b *bufferedFile) Read(bytes []byte) (int, error) {
@@ -25,9 +36,26 @@ func (b *bufferedFile) Close() error {
 	return nil
 }
 
-func newBufferedFile(buf io.Reader, info fs.FileInfo) *bufferedFile {
-	return &bufferedFile{
-		buf:  buf,
-		info: info,
-	}
+func (b *bufferedFile) Name() string {
+	return b.info.Name()
+}
+
+func (b *bufferedFile) Size() int64 {
+	return int64(b.buf.Len())
+}
+
+func (b *bufferedFile) Mode() os.FileMode {
+	return b.info.Mode()
+}
+
+func (b *bufferedFile) ModTime() time.Time {
+	return b.info.ModTime()
+}
+
+func (b *bufferedFile) IsDir() bool {
+	return false
+}
+
+func (b *bufferedFile) Sys() any {
+	return nil
 }