[DEV] Normalize unnecessary executable bits on Python helper scripts

## Summary
Some tracked files in this repo currently have executable mode (100755) even though they are not executed directly in our workflow.

We run Bash/Fish scripts via ./..., so executable bits on *.sh / *.fish are intentional.
Python scripts are run via uv or python ..., so +x is unnecessary for those files.

## Affected files
dev/scripts/check_translation_keys.py
dev/scripts/update_version_announcement.py
## Why this matters
- Reduces noisy permission-only diffs and review comments.
- Keeps file modes aligned with actual execution patterns.
- Improves repo consistency.
## Proposed fix
Change both files from executable (100755) to regular (100644) and commit the mode change only.

## Acceptance criteria
- Both Python files are 100644 in git.
- No content changes, only permission/mode changes.
- Bash/Fish scripts remain executable where needed.

Check the tracked modes of dev/scripts/check_translation_keys.py and dev/scripts/update_version_announcement.py in Git before making any changes. Change only those files from 100755 to 100644, then verify the diff contains permission changes only and that the Bash and Fish scripts remain executable.

Written by the indexing model from the issue text.