Topological editing: fix vertex tool (Z support)

[lbartoletti]

Description

Fix topological point and add z support.

The Z snapped on a segment is always the default Z. I will correct this behavior in a dedicated PR.

Before

After

Sponsored by QWAT group @ponceta

Checklist

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include fixes #11111 in the commit message next to the description
• Commits which add new features are tagged with [FEATURE] in the commit message
• Commits which change the UI or existing user workflows are tagged with [needs-docs] in the commit message and contain sufficient information in the commit message to be documented
• I have read the QGIS Coding Standards and this PR complies with them
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit

Backport

• I request the backport of the changes to the Latest Release
• I request the backport of the changes to the Long Term Release

[backporting]

The backport to release-3_8 failed:

Commits ["e4191a63e6978e70dd725d10758183f8b8594f15","a3c2cf59ca40fea02e4bc9f16f06f8d12ccf9401","cf6cb6bd758a5e3a1f01e7e565bd4ac84d111155","f113898b8c24b95c7952042b37ed72b604c9ab3b","b15361ae17bb80382b91d9a096c700c8b9a885fa","99d8b6bac61ecd066d767c859f59b98e5cd58f99"] could not be cherry-picked on top of release-3_8

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub.
git fetch
# Create new working tree.
git worktree add .worktrees/backport release-3_8
# Navigate to the new directory.
cd .worktrees/backport
# Cherry-pick all the commits of this pull request and resolve the likely conflicts.
git cherry-pick e4191a63e6978e70dd725d10758183f8b8594f15 a3c2cf59ca40fea02e4bc9f16f06f8d12ccf9401 cf6cb6bd758a5e3a1f01e7e565bd4ac84d111155 f113898b8c24b95c7952042b37ed72b604c9ab3b b15361ae17bb80382b91d9a096c700c8b9a885fa 99d8b6bac61ecd066d767c859f59b98e5cd58f99
# Create a new branch with these backported commits.
git checkout -b backport-31163-to-release-3_8
# Push it to GitHub.
git push --set-upstream origin backport-31163-to-release-3_8
# Go back to the original working tree.
cd ../..
# Delete the working tree.
git worktree remove .worktrees/backport

Then, create a pull request where the base branch is release-3_8 and the compare/head branch is backport-31163-to-release-3_8.