config: add CN2 Clinic endpoint

[tao1020]

Summary

• add CN2 as a Clinic upload region pointing to https://clinic.pingkai.cn
• keep CN pointing to https://clinic.pingcap.com.cn
• update CLI and Helm chart region prompts to mention CN2
• sort dynamically loaded regions so the generated region info is deterministic

Validation

• GOOS=darwin GOARCH=arm64 CGO_ENABLED=0 go test ./...
• GOOS=darwin GOARCH=arm64 CGO_ENABLED=0 make check partially passed fmt, vet, and lint; check-static did not run because the Makefile installer could not find golangci-lint v2.10.1
• CN2 upload/rebuild was validated end-to-end against Clinic CN2, with data visible in the Clinic browser panel

[pingcap-cla-assistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 8.46%. Comparing base (82f9593) to head (675fa2f).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/config/info.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #497      +/-   ##
=========================================
- Coverage   10.70%   8.46%   -2.24%     
=========================================
  Files         124     124              
  Lines       12591   10448    -2143     
=========================================
- Hits         1347     884     -463     
+ Misses      11073    9409    -1664     
+ Partials      171     155      -16     

Flag	Coverage Δ
unittests	8.46% <0.00%> (-2.24%)	⬇️

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