sse: add closed SelectCase to handle request closing signal

[wuhan005]

Describe the pull request

A clear and concise description of what the pull request is about, i.e. what problem should be fixed?

Link to the issue: flamego/flamego#187

Checklist

• I agree to follow the Code of Conduct by submitting this pull request.
• I have read and acknowledge the Contributing guide.
• I have added test cases to cover the new code.

[Copilot]

Pull Request Overview

This pull request adds a new "closed" select case to the Server-Sent Events (SSE) connection handler to properly manage request closure signals. Key changes include:

• Adding a new test case for handling connection closure in sse_test.go.
• Updating the Bind function and connection.handle method in sse.go to process a cancellation signal.
• Extending the select cases in connection.handle to include a closed case that returns immediately.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
sse_test.go	New test added to simulate and verify behavior on connection close.
sse.go	Modified Bind and connection.handle to include closed signal handling.

Comments suppressed due to low confidence (1)

sse_test.go:162

• [nitpick] Instead of using a fixed sleep duration to wait for potential events after the connection is closed, consider using synchronization primitives (such as a WaitGroup or channel notifications) to deterministically verify that no events are delivered.

time.Sleep(3 * time.Second)