fix: handle stream closed error with reconnection

[ntalfer]

Summary

Adds proper error handling for {:error, {:stream_error, :closed}} to gracefully reconnect when the SSE server closes the stream.

Problem

During load testing, we observed errors like:

09:37:30.358 [error] user_c2f88c99-7612-419c-a573-b845b1471862 / 5009 ms / 0 < 192 / 0:  Unexpected message {:error, {:stream_error, :closed}}

This error occurred when:

1. The SSE server timeout (default 5 minutes) was shorter than the client timeout
2. The server closed the stream after its timeout
3. The error fell through to the catch-all "Unexpected message" handler
4. The connection failed and error metrics were incremented

Solution

Add a specific error handling clause for {:error, {:stream_error, :closed}} that:

• Calls the reconnect/3 function when the stream is closed
• Respects the SSE_AUTO_RECONNECT configuration
• Allows seamless connection resumption with Last-Event-ID support
• Prevents false error metrics for expected reconnections

Changes

• Added pattern match for {:error, {:stream_error, :closed}} in wait_for_messages/2
• Routes to existing reconnect/3 function with appropriate reason message

Test Plan

• With SSE_AUTO_RECONNECT=true, connections now gracefully reconnect on stream closure
• Error metrics (users with status error) are no longer incorrectly incremented
• Reconnection metrics properly track expected reconnections

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