feat(antigravity): add payload config support to Antigravity executor

Add applyPayloadConfig calls to all Antigravity executor paths (Execute,
executeClaudeNonStream, ExecuteStream) to enable config.yaml payload
overrides for Antigravity/Gemini-Claude models.

This allows users to configure thinking budget and other parameters via
payload.override in config.yaml for models like gemini-claude-opus-4-5*.