fix: use indices:dots arrayFmt in MarshalRoot for correct multipart array encoding

Changes arrayFmt from "comma" to "indices:dots" in MarshalRoot so that
arrays of objects produce indexed field names (e.g. files.0.dest_path)
instead of ambiguous repeated names (e.g. files.dest_path).

This only affects the two MarshalMultipart callers that have array
fields (BrowserFUploadParams, BrowserLoadExtensionsParams). The four
callers without array fields are unaffected since the array encoder
is never invoked. The objKeyEncoder also behaves identically for both
formats (both use dot notation for nested objects).

Made-with: Cursor

Author: rgarcia

internal/apiform/encoder.go

@@ -30,7 +30,7 @@ func MarshalRoot(value any, writer *multipart.Writer) error {
 	e := &encoder{
 		root:       true,
 		dateFormat: time.RFC3339,
-		arrayFmt:   "comma",
+		arrayFmt:   "indices:dots",
 	}
 	return e.marshal(value, writer)
 }