Part of #1592

Scope

Implement GET /v1beta/models for model listing and integration with TUI model picker.

Files to Modify

• crates/zeph-llm/src/gemini.rs -- implement list_models_remote()
• crates/zeph-llm/src/any.rs -- add AnyProvider::Gemini arm to list_models_remote() match

Key Implementation Details

• Endpoint: GET /v1beta/models?key={API_KEY}
• Response contains model list with name, displayName, inputTokenLimit, outputTokenLimit
• Map to RemoteModelInfo { id, display_name, context_window, created_at }
• Filter to generative models only (exclude embedding-only models from chat list)

Acceptance Criteria

• list_models_remote() returns available Gemini models
• Context window populated from inputTokenLimit
• Display name populated from API response
• Model cache integration works
• Unit tests with wiremock