[Feature]: Support Custom Base URL Configuration for GeminiChatModel

[caichicong]

Feature Request: Support Custom Base URL Configuration for GeminiChatModel

Issue Description

Currently, the GeminiChatModel class does not support configuring a custom base URL, which limits flexibility for developers who need to:

1. Use proxy servers / API gateways to forward requests to Google Gemini/Vertex AI services
2. Connect to private, on-premises, or regional API endpoints
3. Test with mock services or local emulators

GeminiChatModel.builder()
                    .apiKey(apiKey)
                    .modelName(modelName)
                    .baseUrl(baseUrl)
                    .build();

[rrrjqy66]

I opened a draft PR for this: #1174.

This adds a baseUrl(String) convenience method to GeminiChatModel.Builder and maps it to the underlying Google GenAI HttpOptions.

The main reason for doing it this way is that the Google SDK already supports endpoint override at the HTTP options layer, but AgentScope-Java currently only exposes the lower-level httpOptions(...) entrypoint. That makes Gemini less ergonomic than the other model builders for reverse proxies, private gateways, and mock endpoints.

I kept the first PR focused on the core builder API plus docs so the change stays small and easier to review. More advanced transport or proxy configuration remains available through httpOptions(...) / clientOptions(...).