fix: Added changes to handle headers as case-insensitive

[shivaygupta-dotcom]

GH: Kong/deck#2080

Fixed three corner cases in header-based routing.
When a header's enum had duplicate values differing only by case (e.g. us-east vs US-EAST), or the same header was defined twice with different casing (e.g. X-Env and x-env), decK was generating redundant or conflicting routes.

We now normalize and deduplicate header names and values, merge same-header definitions, and filter out empty string enum values — resulting in cleaner and more predictable route generation

slack thread: https://kongstrong.slack.com/archives/C04349E4KRC/p1779423079760519

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.75000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.50%. Comparing base (04e8549) to head (f026624).

Files with missing lines	Patch %	Lines
openapi2kong/openapi2kong.go	90.90%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #298      +/-   ##
==========================================
+ Coverage   65.07%   65.50%   +0.43%     
==========================================
  Files          25       25              
  Lines        3204     3256      +52     
==========================================
+ Hits         2085     2133      +48     
- Misses        905      907       +2     
- Partials      214      216       +2     

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