feat: reserve GROUPS in the native window-frame protocol

[manuzhang]

Which issue does this PR close?

Related to #4836. This PR does not close it because supported Spark versions cannot yet construct GROUPS window frames.

Rationale for this change

DataFusion supports GROUPS frames, but Apache Spark versions supported by Comet expose only ROWS and RANGE in both the SQL parser and the sealed Catalyst FrameType. This PR adds forward-compatible protocol and native-planner groundwork without claiming that Spark SQL can originate GROUPS frames today.

What changes are included in this PR?

• Add a reserved Groups value to the window-frame proto enum.
• Convert incoming GROUPS protocol frames to DataFusion WindowFrameUnits::Groups.
• Use unsigned physical offsets for GROUPS, matching the protocol convention used by ROWS.
• Keep Spark window-frame serialization unchanged until Spark exposes a reachable GROUPS representation.
• Add focused native tests for protocol conversion of GROUPS offsets and default unbounded bounds.

How are these changes tested?

• cargo test -p datafusion-comet test_groups_window_frame --lib
• ./mvnw -Pjdk17 -DskipTests compile
• The follow-up scope correction was independently reviewed; no additional tests were run.