TODO: last event is not final error in a2a remote agent call

[vadimi-ep]

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Describe the Bug:
While using a2a remote agent with streaming mode call I often get TODO: last event is not final error. The code is pretty simple:

	runcfg := agent.RunConfig{
		StreamingMode: agent.StreamingModeSSE,
	}
	for resp, err := range arunner.Run(ctx, userID, req.SessionID, msg, runcfg) {
		// err here is "TODO: last event is not final"
	}

Expected Behavior:
No error should be returned

Observed Behavior:
receiving TODO: last event is not final error

Environment Details:

• ADK Library Version: v0.5.0
• OS: Linux, macos
• Go Version: 1.26

Model Information:

• Which model is being used: gemini-3-flash-preview

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Regression:
Did this work in a previous version of ADK? (Yes/No) If so, which one?
No, it was happening in version 0.4.0 too

How often has this issue occurred?:

• Often (50%+)