fix: github_copilot_sdk.py - prioritize BYOK_MODELS over API fetch when explicitly configured

[silenceroom]

Summary

• Prioritize BYOK_MODELS over API fetch when explicitly configured — if a user sets BYOK_MODELS, the plugin now uses that list directly and skips the /models API call to BYOK_BASE_URL
• Remove the now-redundant fallback block that parsed BYOK_MODELS only after API fetch failed

Behavior Change

Scenario	Before	After
Only BYOK_BASE_URL set	API fetch	API fetch
Both set	API fetch first, fallback to BYOK_MODELS on failure	BYOK_MODELS first, API fetch skipped
Neither set	Empty list	Empty list

Breaking change for users who have both BYOK_BASE_URL and BYOK_MODELS configured: the dynamic model list from API fetch will no longer be used. If you want the API-fetched list, clear BYOK_MODELS.

Test Plan

• Configure only BYOK_BASE_URL — verify models are fetched via API as before
• Configure both BYOK_MODELS and BYOK_BASE_URL — verify BYOK_MODELS takes precedence

[gemini-code-assist]

Code Review

此拉取请求修改了 _fetch_byok_models 函数，通过在尝试 API 获取之前优先检查并解析用户配置的 BYOK_MODELS，优化了模型加载逻辑。审查建议改进对 effective_models 的校验逻辑，因为仅使用 .strip() 可能无法处理仅包含分隔符的字符串，建议先解析列表并根据解析后的结果是否为空来决定是否跳过 API 获取。

[gemini-code-assist]

The current logic checks effective_models.strip() to decide whether to skip the API fetch. However, if effective_models contains only delimiters (e.g., ", ,"), model_list will be empty, yet the API fetch will still be skipped because the condition is truthy. It is more robust to parse the models first and only skip the API fetch if the resulting list is non-empty.

        model_list = [m.strip() for m in effective_models.split(",") if m.strip()]
        if model_list:
            await self._emit_debug_log(
                f"BYOK: Using user-configured BYOK_MODELS ({len(model_list)} models)."
            )
        elif effective_base_url: