Fix subagent tool calling loops silently switching models

[Copilot]

Subagents were silently falling back to GPT-4.1 when endpoint.supportsToolCalls returned false, causing tools to be unavailable on the user's selected model (e.g., Gemini 3 Pro). This resulted in sporadic failures with "Server error. Stream terminated" as models couldn't access required tools.

Changes

• Removed model switching logic from three subagent tool calling loops:

  • SearchSubagentToolCallingLoop
  • CodebaseToolCallingLoop
  • McpToolCallingLoop

• Added unit tests verifying subagents preserve the parent request's model regardless of supportsToolCalls value

Before

private async getEndpoint(request: ChatRequest) {
    let endpoint = await this.endpointProvider.getChatEndpoint(this.options.request);
    if (!endpoint.supportsToolCalls) {
        endpoint = await this.endpointProvider.getChatEndpoint('gpt-4.1');
    }
    return endpoint;
}

After

private async getEndpoint(request: ChatRequest) {
    return await this.endpointProvider.getChatEndpoint(this.options.request);
}

Subagents now consistently use the same model as their parent request. If a model genuinely lacks tool support, it will fail explicitly rather than silently switching models.

Original prompt

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This section details on the original issue you should resolve

<issue_title>SubAgents that are created with Gemini 3 Pro model are not getting any tools</issue_title>
<issue_description>
Type: Bug

Every time when I am making requests using the Gemini 3 Pro model the subAgents that are created in that request won't get any tools and every request is failing or the models goes haywire as it cannot follow the instructions as the required tools are missing for it to access.

VS Code version: Code - Insiders 1.108.0-insider (7f08f95ad54782bd242f5536470b330282197333, 2025-12-19T14:09:25.917Z)
OS version: Linux x64 6.2.10-060210-generic
Modes:

This is sporadic and always not the case because yesterday I had the issue for some time window, some requests worked and Today also its the same and I felt I need to report it as a bug

It's always this error:

Comments on the Issue (you are @copilot in this section)

• Fixes SubAgents that are created with Gemini 3 Pro model are not getting any tools vscode#284615

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