antigravity: bump google-antigravity 0.1.5 -> 0.1.7

[joycel-github]

Summary

• Bump google-antigravity 0.1.5 → 0.1.7 in the sidecar requirements.
• 0.1.7 reads GenAI credential env vars natively; this is the prerequisite for removing the sidecar's _vertex_kwargs_from_env override (follow-up PR).
• Behavior unchanged here: with the override still present, its explicit kwargs take precedence, so the result is identical.

Validation

• Sidecar test suite: 32/32 pass against 0.1.7.
• Exercised real 0.1.7 locally: bare LocalAgentConfig() + env vars routes to Vertex with project/location hydrated (also via USE_ENTERPRISE); AI Studio path unchanged.
• Reviewed the 0.1.5→0.1.7 SDK diff scoped to ax's surface (Agent, LocalAgentConfig, types.{Text,Thought,ToolCall}, conversation.chat) — no breaking changes; new LocalAgentConfig params are additive.

Part of #325.

Next:

• clean up overrides that no longer needed with 0.1.7