[Processing] Fix regression in "Generate XYZ tiles (MBTiles)" algorithm (Fix #59986)

[agiudiceandrea]

Description

This PR fixes a regression introduced in the "Generate XYZ tiles (MBTiles)" (native:tilesxyzmbtiles) processing algorithm since QGIS 3.34.0, probably an oversight in #54321 where it was forgotten to add the "description" key to the metadata table.

This PR restores the previous behaviour (which was based on GDAL https://github.com/OSGeo/gdal/blob/f314d4b6bc0b70582fd1cb07f8ae95d01e9c8a86/frmts/mbtiles/mbtilesdataset.cpp#L3070-L3076) just re-adding the missing "description" key to the metadata table.

Fixes #59986.
See also https://www.reddit.com/r/UAVmapping/comments/1c651ze/comment/l014q5n/.

[github-actions]

🪟 Windows builds

Download Windows builds of this PR for testing.
Debug symbols for this build are available here.
(Built from commit 62fd011)

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
(Built from commit 62fd011)

[rouault]

(it seems we have no tests for that algorithm, but that change is obviously OK)

[qgis-bot]

The backport to release-3_34 failed:

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

stderr

error: could not apply 62fd01177df... [processing] Fix regression in native:tilesxyzmbtiles alg
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 advice.mergeConflict false"

stdout

Auto-merging src/analysis/processing/qgsalgorithmxyztiles.cpp
CONFLICT (content): Merge conflict in src/analysis/processing/qgsalgorithmxyztiles.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-release-3_34 release-3_34
# Navigate to the new working tree
cd .worktrees/backport-release-3_34
# Create a new branch
git switch --create backport-60001-to-release-3_34
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 62fd01177df439fc14d7afc3c6a8cf11a92d5698
# Push it to GitHub
git push --set-upstream origin backport-60001-to-release-3_34
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_34

Then, create a pull request where the base branch is release-3_34 and the compare/head branch is backport-60001-to-release-3_34.