Fix single band grey widget min/max live editing

[elpaso]

Fix #65566 by not updating the renderer if the values being edited in the widgets are not valid according to the QgsDoubleValidator.

Also fix unreported issues with QgsDoubleValidator not escaping decimal separator and minus sign.

[github-actions]

🍎 MacOS Qt6 builds

Download MacOS Qt6 builds of this PR for testing.
This app is not notarized, run sudo xattr -d com.apple.quarantine /Applications/QGIS*.app to avoid the warning
(Built from commit 337dc9c)

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
To execute locally, unzip the downloaded zip file and run bin\qgis-bin.exe in the extracted directory.
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(Built from commit 337dc9c)

[qgis-bot]

The backport to queued_ltr_backports failed:

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

stderr

error: could not apply 337dc9c0fe4... Fix single band grey widget min/max live editing
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/gui/qgsdoublevalidator.cpp
CONFLICT (content): Merge conflict in src/gui/qgsdoublevalidator.cpp
Auto-merging src/gui/qgsdoublevalidator.h
Auto-merging src/gui/raster/qgssinglebandgrayrendererwidget.cpp
Auto-merging tests/src/gui/testqgsdoublevalidator.cpp

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-queued_ltr_backports queued_ltr_backports
# Navigate to the new working tree
cd .worktrees/backport-queued_ltr_backports
# Create a new branch
git switch --create backport-66619-to-queued_ltr_backports
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 337dc9c0fe4b64bdb9d49c4400910d77a13339ca
# Push it to GitHub
git push --set-upstream origin backport-66619-to-queued_ltr_backports
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-queued_ltr_backports

Then, create a pull request where the base branch is queued_ltr_backports and the compare/head branch is backport-66619-to-queued_ltr_backports.