fix: SSE handler blocking

[wuqinqiang]

#5182
#5155
#5094

1. The close(client) call is placed in the top-level http.HandlerFunc's defer, which means the channel is only closed when the entire HTTP request handler returns.
2. The actual event generation (l.{{.Call}}) runs in a separate threading.GoSafeCtx goroutine. When this goroutine finishes sending all events, the client channel remains open because close(client) is not executed within its scope.
3. The main for loop, which reads from the client channel, will then block indefinitely on case data := <-client: because it never receives a "closed" signal, even though no more events will be sent.
4. Furthermore, the case data := <-client: lacks an ok check, which is a standard Go practice to detect if a channel has been closed, preventing unnecessary processing of zero values.

This results in:

• Goroutine Leak: The main for loop goroutine remains active indefinitely, consuming resources, even after all events have been sent.
• Blocking Behavior: SSE connections do not terminate gracefully after event completion, holding clients in a waiting state.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Copilot]

Pull Request Overview

Fix SSE handler blocking by ensuring the producer goroutine closes the channel and the consumer loop exits cleanly when the stream ends.

• Move close(client) into the producer goroutine to signal completion.
• Add ok check on channel receive to exit the handler when the channel is closed.

[Copilot]

[nitpick] Since the select only has a single receive case, consider simplifying to a for-range loop over the channel to remove the manual ok check and make the control flow clearer: for data := range client { ... }.

[Copilot]

Please add a test that generates an SSE handler and verifies it exits when the producer completes (channel close) and does not block or leak goroutines. This change introduces important lifecycle behavior that should be covered by a test (e.g., a mocked logic that sends N events then returns, asserting the handler returns and no goroutines remain running).