Fix hx-prompt Unicode header encoding

[cyphercodes]

Description

URI-encodes the HX-Prompt header value set by the hx-prompt extension so Unicode prompt responses can be used with fetch without triggering the non ISO-8859-1 header-value error.

Corresponding issue: Fixes #3841

Testing

• npx web-test-runner --files test/tests/ext/hx-prompt.js --browsers chromium --config test/web-test-runner.config.mjs --playwright --coverage=false
• npm test

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (four-dev for this htmx 4 beta bugfix)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded