Add `azd ai agent code download` command

[v1212]

Summary

Add a new command azd ai agent code download that allows developers to download the deployed source code of a code-based hosted agent from Azure AI Foundry.

Motivation

Currently azd deploy pushes local code to Foundry (via CreateAgentFromZip/UpdateAgentFromZip), but there is no reverse operation. Developers cannot:

• Inspect the code that is actually running on a deployed agent
• Restore/clone an agent's source code to local
• Download a previous version's code for comparison or rollback

The Vienna backend already supports this via GET /agents/{name}/code:download?agent_version=N (feature-gated by CodeAgents=V1Preview,HostedAgents=V1Preview).

Proposed Command

azd ai agent code download [name] [flags]

Flags

Flag	Required	Description
[name]	No	Agent name (resolved from azure.yaml if omitted)
--version, -v	No	Agent version to download (default: latest)
--output, -o	No	Output path (default: ./<agent-name>/)
--zip	No	Save as .zip instead of extracting

Behavior

1. Calls GET /agents/{name}/code:download?agent_version=N
2. Default: downloads zip, extracts to output directory, deletes temp zip
3. --zip: saves zip file directly
4. If output path already exists: error with suggestion to use -o
5. Verifies SHA256 hash from x-ms-code-zip-sha256 response header
6. Prints summary: agent name, version downloaded, output path

Implementation

• New API method DownloadAgentCode() in operations.go (streaming pattern, same as DownloadSessionFile)
• New command group code with download subcommand in cmd/code.go
• Register in root.go
• Feature header: CodeAgents=V1Preview,HostedAgents=V1Preview (same as zip upload)

Notes

• No breaking changes to existing commands
• No conflicts with existing command names
• The CodeAgents=V1Preview feature flag is already used by zip upload (deploy), so this is gated at the same level