[processing] Use safer provider://source syntax in more places

[nyalldawson]

Use the safer method of explicitly including the provider type when storing layer sources to string in more places, which eg fixes running the copied qgis_process string when a non gdal layer source is used for a raster layer. Also fixes the same situation when restoring an algorithm from the history log.

Fixes #54264

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit 1636219)

[qgis-bot]

The backport to release-3_34 failed:

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

stderr

error: could not apply 16362196e72... [processing] Use safer provider://source syntax in more places
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".

stdout

CONFLICT (modify/delete): python/PyQt6/core/auto_generated/processing/qgsprocessingutils.sip.in deleted in HEAD and modified in 16362196e72 ([processing] Use safer provider://source syntax in more places).  Version 16362196e72 ([processing] Use safer provider://source syntax in more places) of python/PyQt6/core/auto_generated/processing/qgsprocessingutils.sip.in left in tree.
Auto-merging python/core/auto_generated/processing/qgsprocessingutils.sip.in
Auto-merging src/core/processing/qgsprocessingparameters.cpp
Auto-merging src/core/processing/qgsprocessingutils.cpp
Auto-merging src/core/processing/qgsprocessingutils.h
Auto-merging tests/src/analysis/testqgsprocessing.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-56175-to-release-3_34
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 16362196e72ca3ffb02187902767d3d3ec3ab064
# Push it to GitHub
git push --set-upstream origin backport-56175-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-56175-to-release-3_34.