Fix handling of multiple consecutive tabs with HTML text rendering

[nyalldawson]

Refs #60098

[github-actions]

🪟 Windows builds

Download Windows builds of this PR for testing.
Debug symbols for this build are available here.
(Built from commit c2782ff)

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
(Built from commit c2782ff)

[qgis-bot]

The backport to release-3_40 failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply c2782ffed81... Fix handling of multiple consecutive tabs with HTML text rendering
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config set advice.mergeConflict false"

stdout

Auto-merging src/core/textrenderer/qgstextdocument.cpp
Auto-merging tests/src/python/test_qgstextrenderer.py
CONFLICT (content): Merge conflict in tests/src/python/test_qgstextrenderer.py

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release-3_40 release-3_40
# Navigate to the new working tree
cd .worktrees/backport-release-3_40
# Create a new branch
git switch --create backport-60299-to-release-3_40
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick c2782ffed81be3896360949b2a4fd0da5ac3f0ee
# Push it to GitHub
git push --set-upstream origin backport-60299-to-release-3_40
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_40

Then, create a pull request where the base branch is release-3_40 and the compare/head branch is backport-60299-to-release-3_40.