Surface query failure explicitly in GetAgentInstancesForBlueprintAsync

[Copilot]

GetAgentInstancesForBlueprintAsync was swallowing all Graph/auth exceptions and returning an empty list, making a transient failure indistinguishable from a genuine "0 instances" result — causing cleanup blueprint to silently proceed and potentially recreate orphaned resources.

Changes

• AgentBlueprintService.cs: Removed the try/catch in GetAgentInstancesForBlueprintAsync; exceptions now propagate. Updated XML doc to document throwing behavior.
• AgentBlueprintServiceTests.cs: Added GetAgentInstancesForBlueprintAsync_Throws_WhenGraphQueryFails to lock in propagation behavior.

CleanupCommand already had the correct abort-on-failure handler — it just wasn't reachable:

// Was unreachable before — exception was eaten by the service
catch (Exception ex)
{
    logger.LogError(ex, "Failed to query agent instances for blueprint {BlueprintId}. Aborting cleanup.", config.AgentBlueprintId);
    return;
}

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