Move editToolbar and fix overlapping annotations with it

[LoukasPap]

Fixes #21643

I moved the editToolbar up to page level and added isolation: isolate to .annotationLayer, .annotationEditorLayer and .editor toolbar elements so that z-index works at their level. That way we avoid the z-index of a nested annotation (e.g. a linkAnnotation) "compete" with the z-index of the editToolbar.

This PR also fixes a bug where a textAnnotation was rendered above the editToolbar.

Tests

I used document.elementFromPoint(x,y) to check every time which elements is rendered on top.
I also moved createFreeTextEditor() to test_utils.mjs to make it available to all tests that may need it

Test 1 — "the edit toolbar is rendered above link annotations"

1. Load the PDF at 120% zoom, wait for the text layer to be ready.
2. Find the link pointing to https://github.com/mozilla/pdf.js and get its position.
3. Check what's on top at that spot right now — confirm it's the link itself (pdfjs_internal_id_14R), i.e. nothing is covering it yet.
4. Double-click on "A:" to select it, which makes the floating toolbar appear.
5. Wait for the toolbar's highlight button to show up.
6. Check what's on top at the link's position again — confirm it's now the highlight button, proving the toolbar renders above the link instead of being hidden behind it.

Test 2 — "the edit toolbar is rendered above text annotations"

1. Switch into FreeText editing mode.
2. Find "My text" on the page and note its position.
3. Create a new text annotation containing "CHECK", placed just below "My text".
4. Check what's on top at the middle of that new "CHECK" box — confirm it's the text annotation itself (its own edit toolbar/label), i.e. nothing else is covering it yet.
5. Close the text editor (exit FreeText mode).
6. Double-click on "My text" to select it, which makes the floating toolbar appear.
7. Wait for the toolbar's highlight button to show up.
8. Check what's on top at the toolbar's position — confirm it's the highlight button.
9. Re-open the text editor (re-enter FreeText mode) without closing the floating toolbar, so both are visible on the page at the same time.
10. Check what's on top at the freetext's position — confirm it's still the highlight button, proving the toolbar stays rendered above the reopened text annotation rather than getting covered by it.

[LoukasPap]

This is currently a draft because I haven't yet added the integration tests. I pushed anyway to make sure we agree with the solution and then move on.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.75%. Comparing base (e051279) to head (356901b).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/display/editor/tools.js	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #21723       +/-   ##
===========================================
- Coverage   90.19%   75.75%   -14.44%     
===========================================
  Files         264      264               
  Lines       67324    67327        +3     
===========================================
- Hits        60721    51005     -9716     
- Misses       6603    16322     +9719     

Flag	Coverage Δ
browsertest	66.33% <0.00%> (+0.01%)	⬆️
fonttest	8.91% <ø> (ø)
integrationtest	25.15% <75.00%> (-44.24%)	⬇️
unittest	58.35% <0.00%> (-0.01%)	⬇️
unittestcli	56.91% <0.00%> (-0.01%)	⬇️

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