Changes

feature(pipelines/pingcap/tidb): update MySQL test pipelines for next-gen support (#3606)

- Rename 'MySQL Tests' stage to 'Tests'
- Add TARGET_BRANCH_PD and TARGET_BRANCH_TIKV variables
- Download and cache component binaries in next-gen pipeline
- Add STORE axis for test matrix (initially 'unistore' only)
- Improve test execution and caching logic for next-gen pipeline

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
 (commit: 8f3d162)