Add POT source path validation to per-project processing

These are some of the changes that this MR makes

validate_paths.bash (new file)

Takes Aviral's validation script and modifies it

Key changes:

• Output now matches Scripty's existing log format seen here: https://logs.l10n.kde.org/260323.branches_stable_l10n-kf6
• One WARNING in <file>: <message> line per bad path
• One self-contained summary line per POT always printed
• No output beyond the summary when everything is correct

extract-messages.sh (modified)

Added the following after the repack step, before cleanup:

# Validate that #: source paths in each generated POT are relative to the
# project root. One WARNING line per bad path; one summary line per POT.
# Output goes to stdout so it appears in the Scripty log like other warnings.
find -L $podir -iname '*.pot' | while read -r pot; do
    bash "$l10nscripts/validate_paths.bash" "$pot" "$project_root_dir"
done

Testing

Tested in a Linux Docker container against konsole.

Clean run:

echo '#: src/colorscheme/RandomizationRange.cpp:1' > /tmp/test_clean.pot
bash validate_paths.bash /tmp/test_clean.pot ~/kde/src/konsole
# POT source path check passed: /tmp/test_clean.pot (1 path(s) verified)
# Exit: 0

Real project with known bad paths:

bash validate_paths.bash konsole/po/de/konsole.po ~/kde/src/konsole
# 77 WARNING lines for non-root-relative paths
# Exit: 1

Injected missing path:

echo "#: fake/doesnotexist.cpp:1" >> konsole.po
bash validate_paths.bash konsole.po ~/kde/src/konsole
# WARNING in ...: source path not found in project: fake/doesnotexist.cpp
# Exit: 1