refactor(admin): route tier config manager through AppContext

[overtrue]

Type of Change

• New Feature
• Bug Fix
• Documentation
• Performance Improvement
• Test/CI
• Refactor
• Other:

Related Issues

• feature: support kms && encryt #573

Summary of Changes

• Add TierConfigInterface in AppContext with default adapter to centralize tier-config dependency access.
• Replace direct GLOBAL_TierConfigMgr reads/writes in admin/handlers/tier.rs with AppContext-based access.
• Keep tier admin API behavior unchanged; only dependency wiring moved behind context interfaces.

Checklist

• I have read and followed the CONTRIBUTING.md guidelines
• Passed make pre-commit
• Added/updated necessary tests
• Documentation updated (if needed)
• CI/CD passed (if applicable)

Impact

• Breaking change (compatibility)
• Requires doc/config/deployment update
• Other impact:
• Internal refactor only; no external API changes expected.

Additional Notes

• Validation executed locally via make pre-commit (fmt/clippy/check/test).
• This is a follow-up P5-02 convergence step after PR refactor(app): converge lower-priority global reads via AppContext #1958.

---

Thank you for your contribution! Please ensure your PR follows the community standards (CODE_OF_CONDUCT.md) and sign the CLA if this is your first contribution.

[Copilot]

Pull request overview

This PR refactors tier configuration management to route through AppContext instead of direct global access, continuing the P5-02 convergence initiative started in PR #1958. The changes introduce a new TierConfigInterface boundary in the application context layer and update all tier admin handlers to use it.

Changes:

• Added TierConfigInterface to AppContext with default adapter that wraps GLOBAL_TierConfigMgr
• Replaced all direct GLOBAL_TierConfigMgr reads/writes in tier admin handlers with AppContext-based access via helper function
• Removed direct global import from tier handler module while maintaining identical external API behavior

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
rustfs/src/app/context.rs	Added TierConfigInterface trait, TierConfigHandle adapter, integrated into AppContext struct and initialization
rustfs/src/admin/handlers/tier.rs	Added tier_config_mgr_from_context() helper, updated all 7 operations (Add/Edit/List/Remove/Verify/GetInfo/Clear) to use context-based access, removed GLOBAL_TierConfigMgr import