Add Nemotron-H speculative decoding support (#2145)

* Add Nemotron-H speculative decoding support

* Forward projected_context through DFlash2 and Gemma4 DSpark decoder layers

DFlashDecoderLayer.__call__ gained a projected_context argument, but the
DFlash2 and Gemma4 DSpark layers override __call__ and still used the
four-argument signature, so the shared _hidden() call raised TypeError.

Gemma4 DSpark has no test covering that path; only DFlash2 surfaced in CI.