This is a cherrypick of #16846 into release-13.0.
Revert commit dd2cfa8 (#16685) to undo
the renaming from Gemini Enterprise Agent Platform to Gemini Enterprise
API.
- Restore Backend.agentPlatform(location:) as the primary public API
and remove Backend.enterprise(location:).
- Revert APIConfig.Service.enterprise to .agentPlatform and restore
agentPlatformStagingBypassProxy.
- Revert documentation comments, DocC topics, and guides back to
Agent Platform Gemini API and Gemini Enterprise Agent Platform.
- Clean up leftover Vertex AI symbols to align with Agent Platform and
FirebaseAILogic (isFirebaseAILogicServiceDisabledError and
defaultAgentPlatformAPIConfig).
- Re-align tests and conflict resolutions from #16689, #16693, #16676,
and #16756.