Add trigger 'htmx:sseClose' to sse close event

[sondalex]

This PR adds a new trigger, "htmx:sseClose", which is emitted when an SSE closing message is received.

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[Telroshan]

Hey, while I like the idea, it seems you aren't firing this event in all cases where a source might be closed, was it intended?
For ex I can see those places in the code where you didn't apply it:

htmx-extensions/src/sse/sse.js

Lines 47 to 49 in 8845604

	if (internalData.sseEventSource) {
	internalData.sseEventSource.close()
	}

htmx-extensions/src/sse/sse.js

Lines 239 to 243 in 8845604

	if (source != undefined) {
	source.close()
	// source = null
	return true
	}

Also, do you think that, on error, a sseClose event should also be fired? (since it can close the event source, cf the reconnection algorithm)

Also, to accept this PR it'd be good to add the following :

• Documentation for the event, as a recent PR (Add section on events dispatched by SSE extension to SSE documentation #25) added documentation for all the events dispatched by this extension, we should keep it up to date
• Add test cases to ensure the event is correctly fired in all closing situations

[sondalex]

Hi, thank you for the review and recommendations! I understand your concerns about not firing the htmx:sseClose event in all cases where a source might be closed.

Initially, I intended to trigger the event only when a closing message is received, not when the connection is closed due to the removal of the element containing sse-connect. However, I see the value in firing the event in all closing situations.

Regarding your suggestion to fire the same event for all closing connections, I'm open to implementing htmx:sseClose for all cases. Alternatively, I could introduce separate events, such as htmx:sseNodeClose for node replacement and htmx:sseMessageClose for closing messages. What are your thoughts on this ?

[Telroshan]

Oh I see, makes sense! I think a parameter could make more sense? An enum-like value in the event's details that lets the user know, if it matters to them, what was the reason of this sse close. So that if someone just wants to bind some logic upon a sse connection closing, they don't have to bind 3 different events, and if they need to differentiate the scenarios, they may simply use if/else or a switch statement

[sondalex]

Hi @Telroshan , I have added test cases and implemented the close types in the event details. Thank you for the recommendation.

Users can access the sseClose details as follow:

document.body.addEventListener('htmx:sseClose', function (e) {
    const reason = e.detail.type
    switch(reason) {
        case "nodeMissing":
            // Parent node is missing and therefore connection was closed
            ...
        case "nodeReplaced":
            // Parent node replacement caused closing of connection 
            ...
        case "message":
            // connection was closed due to reception of message sse-close 
            ...
    }
})

[Telroshan]

Looks good @sondalex, thanks!
Last request before we can merge this ; could you please remove the formatting changes? To concentrate the diff on the actual code changes

[sondalex]

Sure @Telroshan , I have just fixed it.

[Telroshan]

Perfect, thanks!