fix(google-gemini): skip FunctionCallingConfig when only native tools are present

[he-yufeng]

问题

开启 Gemini 原生搜索（google_search）或 URL 上下文（url_context）功能时，如果没有注册任何函数工具，_prepare_query_config 仍然会创建 FunctionCallingConfig，导致 Gemini API 返回 400 INVALID_ARGUMENT：

Function calling config is set without function_declarations.

原因

tool_config 的构建条件 if tools and tool_list 过于宽泛。当 tool_list 中只有原生工具（google_search、url_context、code_execution）而没有 function_declarations 时，也会走进去设置 FunctionCallingConfig，但 Gemini API 不允许在没有 function_declarations 的情况下设置该配置。

修改

在创建 tool_config 前，先检查 tool_list 里是否真的包含 function_declarations，只有在有函数声明时才设置 FunctionCallingConfig。

Fixes #7406

Summary by Sourcery

Bug Fixes:

• Prevent Gemini INVALID_ARGUMENT errors by skipping FunctionCallingConfig when only native tools are configured without any function declarations.

[sourcery-ai]

Hey - I've found 1 issue, and left some high level feedback:

• The new has_func_decl condition drops the previous tools and guard; consider keeping tools and has_func_decl to avoid creating a ToolConfig in edge cases where tool_list is non-empty but tools is unexpectedly None or otherwise inconsistent with tool_list.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The new `has_func_decl` condition drops the previous `tools and` guard; consider keeping `tools and has_func_decl` to avoid creating a `ToolConfig` in edge cases where `tool_list` is non-empty but `tools` is unexpectedly `None` or otherwise inconsistent with `tool_list`.

## Individual Comments

### Comment 1
<location path="astrbot/core/provider/sources/gemini_source.py" line_range="217-221" />
<code_context>

         tool_config = None
-        if tools and tool_list:
+        has_func_decl = tool_list and any(
+            t.function_declarations for t in tool_list
+        )
+        if has_func_decl:
             tool_config = types.ToolConfig(
</code_context>
<issue_to_address>
**suggestion:** Normalize `has_func_decl` to a strict boolean to avoid mixed list/bool types.

`has_func_decl = tool_list and any(...)` can produce either `[]` or a `bool`, which complicates type reasoning. Please cast to a strict boolean, e.g. `has_func_decl = bool(tool_list and any(t.function_declarations for t in tool_list))`, so the variable is always a `bool`.

```suggestion
        tool_config = None
        has_func_decl = bool(
            tool_list and any(t.function_declarations for t in tool_list)
        )
        if has_func_decl:
```
</issue_to_address>

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[gemini-code-assist]

Code Review

This pull request refines the logic for initializing tool_config in the Gemini source by specifically checking for function declarations within the tool list, rather than just checking for the existence of the list itself. I have no feedback to provide as there were no review comments to evaluate.