agentHost: Require explicit SDK download on metered connections

[copilot-swe-agent]

Follow-up to #334316.

Once a user has approved an Agent SDK download, a missing or newer Claude/Codex SDK is downloaded automatically when the agent is used. On a metered connection, that automatic download should not start without an explicit user action.

Expected behavior:

• Detect a metered connection before starting an automatic download-on-use.
• Do not start the SDK download automatically while the connection is metered.
• Surface a setup notification or error that explains why the SDK was not downloaded and provides a Download action so the user can explicitly start it.
• Keep the explicit download path available for each missing or outdated Agent SDK.