fix: honor group validation settings at runtime

[Arvin-Lee]

Fixes #1010.

SkipValidateBody and SkipValidateParams set through group modifiers or OnAddOperation hooks were ignored for prefixed groups because the request handler read the original operation instead of the modified route-specific copy.

This change preserves hook-configured validation flags during route registration and uses each route’s flags at request time. Sibling routes retain independent validation settings.

Regression tests cover:

• Both flags independently and together.
• Group modifiers and OnAddOperation hooks.
• Nested groups and multiple prefixes.
• Hidden routes.

Validation on Go 1.27.1:

• Regression tests fail before the fix and pass afterward.
• GOEXPERIMENT=nojsonv2 go test -race ./... passes.
• With the default Go 1.27 configuration, all packages pass except for four previously observed YAML tests.
• git diff --check passes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.20%. Comparing base (5d2b1be) to head (a471ae6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1121      +/-   ##
==========================================
+ Coverage   93.19%   93.20%   +0.01%     
==========================================
  Files          24       24              
  Lines        5008     5019      +11     
==========================================
+ Hits         4667     4678      +11     
  Misses        273      273              
  Partials       68       68              

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