feat(orbit): add GetDsl for /api/v4/orbit/schema/dsl

Michael Angelo Riverarequested to merge
add-orbit-get-dsl into main

from agent

Adds OrbitService.GetDsl hitting GET /api/v4/orbit/schema/dsl. Body is captured as json.RawMessage so both format=raw (JSON Schema object) and format=llm (TOON string) pass through verbatim.

Relates to gitlab-org/gitlab#591015. Required so glab orbit remote dsl can land in gitlab-org/cli.

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