chore(headless): internalize fetch-event-source

[louis-bompart]

splits from #4921

Replace @microsoft/fetch-event-source.

Essentially a good old copy of the sources, with our patch applied on top and some tweaks in the UT so that they work with Vitest.

Includes a copy of the LICENSE.

[github-actions]

Pull Request Report

PR Title

✅ Title follows the conventional commit spec.

Live demo links

• Storybook

• Playwright report

Bundle Size

File	Old (kb)	New (kb)	Change (%)
case-assist	244.2	243.7	-0.2
commerce	355.5	355	-0.1
search	415.5	415	-0.1
insight	406.7	406.2	-0.1
recommendation	256.3	255.9	-0.2
ssr	409.3	408.8	-0.1
ssr-commerce	373.2	372.7	-0.1

[dmgauthier]

Out or curiosity.
We usually move to "externalize" utils and tools to open source alternative whenever we can. Why the other way around now ?
I read between the line that this is to fix source maps error, is there more to it ?

[mmitiche]

Out or curiosity. We usually move to "externalize" utils and tools to open source alternative whenever we can. Why the other way around now ? I read between the line that this is to fix source maps error, is there more to it ?

More context could be found in this thread: https://coveo.slack.com/archives/G016TA2G485/p1738619735863479

[mmitiche]

LGTM!

[erocheleau]

very nice thank you!

[louis-bompart]

Out of curiosity. We usually move to "externalize" utils and tools to open-source alternatives whenever we can. Why the other way around now? I read between the lines that this is to fix source map errors. Is there more to it?

Essentially, the library is abandoned, and it's causing us issues. So, either we find an alternative library or we incorporate it.
The latter was "cheaper" to do essentially.