Ensure addActivityItemStream() doesn't lose Output vs Error stream specialization

[DavisVaughan]

Addresses #4519

In addActivityItemStream(), either a ActivityItemOutputStream or ActivityItemErrorStream always goes in, but it was possible for a "generic" ActivityItemStream to come out here when we make one from the remainderText after the last newline:

positron/src/vs/workbench/services/positronConsole/browser/classes/activityItemStream.ts

Lines 118 to 136 in e51332d

	// Create the remainder ActivityItemStream.
	activityItemStream = new ActivityItemStream(
	activityItemStream.id,
	activityItemStream.parentId,
	activityItemStream.when,
	remainderText
	);
	// If this ActivityItemStream isn't terminated, push the remainder ActivityItemStream to it
	// and return undefined, indicating that there is no remainder ActivityItemStream to be
	// processed.
	if (!this.terminated) {
	this.activityItemStreams.push(activityItemStream);
	return undefined;
	}
	// Return the remainder ActivityItemStream to be processed.
	activityItemStream.ansiOutput.copyStylesFrom(this.ansiOutput);
	return activityItemStream;

That generic ActivityItemStream would get set here:

positron/src/vs/workbench/services/positronConsole/browser/classes/runtimeItemActivity.ts

Lines 110 to 116 in c9504d3

	const activityItemStream = lastActivityItem.addActivityItemStream(activityItem);
	if (!activityItemStream) {
	return;
	}
	// Set the activity item to add.
	activityItem = activityItemStream;

And then pushed onto this._activityItems

positron/src/vs/workbench/services/positronConsole/browser/classes/runtimeItemActivity.ts

Lines 138 to 139 in c9504d3

	// Push the activity item.
	this._activityItems.push(activityItem);

The problem is that our Console code doesn't know how to deal with a generic ActivityItemStream. It must be a specialized ActivityItemOutputStream or ActivityItemErrorStream!

positron/src/vs/workbench/contrib/positronConsole/browser/components/runtimeActivity.tsx

Lines 46 to 49 in c9504d3

	} else if (activityItem instanceof ActivityItemOutputStream) {
	return <ActivityOutputStream key={activityItem.id} activityItemOutputStream={activityItem} />;
	} else if (activityItem instanceof ActivityItemErrorStream) {
	return <ActivityErrorStream key={activityItem.id} activityItemErrorStream={activityItem} />;

If a generic ActivityItemStream slipped through, the output would never appear in the Console, making it look "dropped", as reported in #4519.

---

I like how we have a generic ActivityItemStream class that we specialize for the output/error types, so I've fixed this bug by using polymorphic this to allow addActivityItemStream() to take and return objects of the correct specialized type.

Ideally we'd simply be able to use new this.constructor() in place of new ActivityItemStream() to use the polymorphic constructor when we need to create new objects, but due to some well known weirdness in typescript, you have to manually cast it to the constructor type, so newActivityItemStream() wraps that up.

I've also made the ActivityItemStream constructor protected to avoid us using it by accident in the codebase. We should always be creating ActivityItemOutputStream or ActivityItemErrorStream instead.

Before 😢

Screen.Recording.2024-09-30.at.2.18.03.PM.mov

After 🥳

Screen.Recording.2024-09-30.at.2.16.33.PM.mov

[softwarenerd]

LG!