fix: Emit an error when the stream closes before any response

[keelerm84]

On Linux, swift-corelibs-foundation (libcurl) terminates a 3xx redirect with an
empty or missing Location header by completing the task with no error and no
response -- no didReceive, no willPerformHTTPRedirection callback. EventSource
treated that as a normal close and reconnected forever, never emitting an error,
so the SSE contract tests "client handles empty/missing Location header with
301/307 status" timed out. (CFNetwork surfaces it as an error, so macOS passed.)

Treat a no-error completion that never opened the stream as an unrecoverable
error and stop, giving deterministic behavior across platforms. Adds a
regression test that finishes a request with no response and asserts an
unrecoverable error with no reconnect.

---

Note

Medium Risk
Changes connection teardown in didCompleteWithError for a narrow edge case; misclassification could stop valid reconnects or still loop, but scope is limited to tasks that never reached .open.

Overview
Fixes infinite reconnect when a URLSession task completes successfully but never delivers headers (so readyState never becomes .open). On Linux/libcurl this happens for 3xx redirects with empty or missing Location; macOS often reports an error instead.

In urlSession(_:task:didCompleteWithError:), a new branch treats no error + never opened as an unrecoverable EventSourceError, sets shutdown, finishes the stream, and skips the reconnect loop.

Adds closeWithoutResponseEmitsUnrecoverableError to finish a mock request without responding and assert a non-recoverable error with no follow-up request.

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