Add antigravity engine, deprecate Gemini (#34693)

* Add antigravity engine and deprecate Gemini engine

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

* Fix duplicate doc comment and improve test name per code review

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

* docs(adr): add draft ADR-34693 for antigravity engine and gemini deprecation

Draft ADR generated by the Design Decision Gate workflow. Documents the
decision to introduce antigravity as a parallel engine to gemini, with
soft-deprecation via compile-time warning rather than rename or hard
removal. PR author should review and finalize before merge.

* Initial plan for addressing PR review comments

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

* Address PR review comments: fix doc comments, alias port constant, use correct deprecated target, strengthen test assertion

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Peli de Halleux <pelikhan@users.noreply.github.com>

Author: 3 people