Extract reusable metadata workflow

[chargome]

Linear ticket: https://linear.app/getsentry/issue/JS-1875

Parent: #19671
PR: #19680

Goal

Move job_get_metadata into reusable-metadata.yml using workflow_call. No behavior change — all downstream output references remain the same.

This is the foundation step. Once metadata is a reusable workflow, each new independent test workflow can call it to get change-detection flags (changed_node_integration, changed_browser_integration, etc.) and branch-type info (is_base_branch, is_release, force_skip_cache).

Status

PR #19680 is open and approved. Needs to be merged to unblock subsequent work.

[linear-code]

JS-1875