test: multi label + update title

[LeVinhGithub]

Summary

Extends the reusable create-pr-feature-flag-registry-drift workflow so callers can pass multiple PR labels (comma- or newline-separated) and a custom PR title prefix, while still appending the existing UTC timestamp suffix.

Changes

• pr-label: Description updated; labels are split on commas/newlines, trimmed, and passed as repeated gh pr create --label arguments.
• pr-title-prefix: New optional workflow_call input (default in this commit: test: Sync Feature Flag Registry) wired into PR_TITLE_PREFIX and used as --title "${PR_TITLE_PREFIX} - $TIMESTAMP".

Testing

• Call the workflow with pr-label as comma-separated and multiline values and confirm all labels apply.
• Call with pr-title-prefix and confirm title is {prefix} - {timestamp}.

Fixes

Fixes: N/A

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Note

Low Risk
Limited to a GitHub Actions workflow and only changes PR metadata (labels/title); main risk is minor label parsing issues affecting labeling.

Overview
Updates the reusable create-pr-feature-flag-registry-drift workflow to support applying multiple PR labels by parsing pr-label as comma/newline-separated values and passing repeated gh pr create --label flags.

Adds a new optional pr-title-prefix input and uses it to build the PR title (${prefix} - ${timestamp}) instead of the previously hardcoded [QA] Sync Feature Flag Registry title.

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