JGC-499 - Remove concurrency from reusable test suites

[ehl-jf]

Summary

Fixes the post-merge Build Gate run on master (#3535), where only frogbot ran and build-gate-success failed immediately because every test suite was cancelled before starting.

Root cause

Each reusable suite still declared its own:

concurrency:
  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.sha }}-${{ github.ref }}
  cancel-in-progress: true

When a workflow is invoked via workflow_call, github.workflow resolves to the caller (Build Gate), so all 25 suites and the parent run compute the same concurrency group. With cancel-in-progress: true they cancelled one another (and collided with the parent) before starting. frogbot survived only because it has no concurrency block. With all suites cancelled, build-gate-success (if: always()) ran and failed on contains(needs.*.result, 'cancelled').

Fix

Remove the concurrency: block from the 25 reusable suite workflows. The orchestrator (build-gate.yml) keeps its own concurrency, which already supersedes older runs per PR/branch — so cancellation behaviour is preserved without the self-collision.

Validation

• All 25 suite files parse as valid YAML.
• Deletions only (105 deletions(-)); no logic changed.