Support Agent Insights JSON export in the azd Agents extension

## Problem

Teams using Microsoft Foundry Agent Insights need a command-line export workflow for PowerShell and CI automation. They need to archive findings, filter them, and pass descriptions, trace evidence, and proposed fixes into engineering workflows without copying portal content manually.

## Proposed solution

Add `azd ai agent insights export [name]` to the `azure.ai.agents` extension:

- Export all result pages as JSON to stdout or a UTF-8 file with `--out-file`.
- Include trace evidence and proposed fixes by default, with category, severity, status, and detail-projection options.
- Support azd service/environment resolution and explicit Foundry project endpoints.
- Keep environment-bound exports in the selected project and preserve unknown service fields.
- Include offline command scenarios and regression coverage for API headers, pagination, environment isolation, and output handling.

Implementation: https://github.com/Azure/azure-dev/pull/9937

Start with the implementation in linked pull request #9937, then review the azure.ai.agents extension entry points and its regression coverage. Verify pagination, API headers, environment isolation, offline scenarios, and stdout or UTF-8 file output against the proposed command behavior.

Written by the indexing model from the issue text.