[CI] Fix logic gap: PO to JSON validation does not run on PRs

[varruunnn]

Description:
Previously, the Convert PO to JSON and verify changes workflow only ran on push, which allowed PRs with out-of-sync .po and generated .json files to pass CI and merge. The validation would only run after the merge, causing failures on the main branch.

To verify the fix, I pushed a test change to es.po. Before this update, the convert-and-verify check passed on the PR even though the generated JSON was out of sync. After adding the pull_request trigger and updating the diff logic, the workflow correctly runs during the PR and fails when the generated files are not updated.

This confirms the workflow now acts as a pre-merge validation check instead of a post-merge failure.