feat: Add Style Linters (usestdlibvars, dupl)

[bupd]

Comprehensive Summary of your change

Add style linters to enforce consistency:

• usestdlibvars - Enforces stdlib constants (eg http.MethodGet, http.StatusOK)
• dupl - Detects duplicate code blocks (threshold: 300 tokens)
• Set dupl threshold to 300 tokens to avoid false positives
• Fixed usestdlibvars violations (replaced literals with constants)
• Added revive exclusion for var-naming in existing packages

Issue being fixed

Fixes #22665

Please indicate you've done the following:

• Well Written Title and Summary of the PR
• Label the PR as needed. "release-note/update"
• Accepted the DCO. Commits without the DCO will delay acceptance.
• Made sure tests are passing and test coverage is added if needed.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed in website repository.

[codecov]

Codecov Report

❌ Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.88%. Comparing base (c8c11b4) to head (6f4bee5).
⚠️ Report is 634 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pkg/oidc/helper.go	0.00%	1 Missing ⚠️
src/pkg/reg/adapter/huawei/huawei_adapter.go	66.66%	1 Missing ⚠️
src/server/v2.0/handler/search.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #22670       +/-   ##
===========================================
+ Coverage   45.36%   65.88%   +20.51%     
===========================================
  Files         244     1074      +830     
  Lines       13333   116357   +103024     
  Branches     2719     2932      +213     
===========================================
+ Hits         6049    76661    +70612     
- Misses       6983    35447    +28464     
- Partials      301     4249     +3948     

Flag	Coverage Δ
unittests	65.88% <70.00%> (+20.51%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/pkg/reg/adapter/huawei/image_registry.go	58.49% <100.00%> (ø)
src/server/registry/referrers.go	61.15% <100.00%> (ø)
src/pkg/oidc/helper.go	43.88% <0.00%> (ø)
src/pkg/reg/adapter/huawei/huawei_adapter.go	55.39% <66.66%> (ø)
src/server/v2.0/handler/search.go	0.00% <0.00%> (ø)

... and 984 files with indirect coverage changes

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[chlins]

lgtm