feat: Report a server-initiated stream close to the connection error handler

[beekld]

Summary

EventSource now treats a close of the stream by the server as a connection failure, and reports it to the ConnectionErrorHandler as a new StreamClosedByServerError. Previously it did not treat that close as a failure, so the handler never ran and could not replace the built-in reconnect.

Because the default ConnectionErrorHandler returns .proceed, a consumer that does not set one now also receives the close through EventHandler.onError.

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Note

Overview
When the SSE stream ends without a URL error (server closes the connection cleanly), the client now treats that as a connection failure and routes it through the same path as other connection errors.

A new public StreamClosedByServerError is introduced for that case. didCompleteWithError is refactored so both transport errors and server-initiated closes set a connectionError, then call dispatchError once—so connectionErrorHandler and (by default) EventHandler.onError run before reconnect. Client-initiated cancels (NSURLErrorCancelled from stop()) are still ignored and do not surface as this error.

Tests expect StreamClosedByServerError on normal stream finish and cover reconnect, .shutdown from the error handler, and that stop() does not report an error.

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