Fix hot-reloading for the {% javascript %} tag when serving compiled assets scripts with multibyte characters

[karreiro]

WHY are these changes introduced?

When hot-reloading compiled JS generated from {% javascript %} blocks, the response Content-Length header could be computed using the string character count rather than the UTF‑8 byte length. This becomes incorrect as soon as the output contains multi-byte characters (e.g., an em dash —).

WHAT is this pull request doing?

This PR updates the compiled_assets script response path to set content-length based on the UTF‑8 byte length.

How to test your changes?

• Run shopify theme dev with a snippet like this:

Hey
{% javascript %}
console.log("Hey, hey")
// ...not full cart — derive count from response

console.log("Hey, hey, heeeey!")
{% endjavascript %}

Post-release steps

n/a

Measuring impact

• n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix
• Existing analytics will cater for this addition
• PR includes analytics changes to measure impact

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes

[frandiox]

Maaakes sense, thanks!

[karreiro]

👋 Hey @Shopify/app-inner-loop, could you please take a look at this when you have a moment?

PS: I've proposed #6818 to prevent similar review requests in the future 🙇

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	78.9%	14554/18446
🟡	Branches	73.23%	7223/9863
🟡	Functions	79.09%	3704/4683
🟡	Lines	79.25%	13761/17364

Test suite run success

3772 tests passing in 1453 suites.

Report generated by 🧪jest coverage report action from 6a7bfce