Add recursion depth check for TYPE_GROUP in UntypedMessage::Decode() (#26467)

UntypedMessage::Decode() in the BinaryToJson code path was missing a call to stream.IncrementRecursionDepth() for WIRETYPE_START_GROUP, allowing unbounded stack recursion via deeply nested groups. The parallel handler for TYPE_MESSAGE in DecodeDelimited() already had this check.

This is the same class of issue as CVE-2024-7254 (protobuf-java).

Fixes https://issuetracker.google.com/issues/493306525

Closes #26467

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/26467 from lonelybones:fix-group-recursion-depth 11f34c388ff97453b21116592b67b0b52e015678
PiperOrigin-RevId: 921655293