Only include relevant snippets

[ciarams87]

Proposed changes

Problem: SnippetsFilters are being included in NGINX configuration for all gateways regardless of whether routes attached to those specific gateways actually referenced the filters.

Solution: Implemented gateway-specific scoping by adding GetReferencedSnippetsFilters() method that returns only SnippetsFilters referenced by routes attached to the current gateway.

Testing: Added comprehensive unit tests for the new scoping logic including positive and negative test cases. Verified all existing tests continue to pass, confirming that the gateway-scoping behavior works correctly and doesn't break existing functionality.

Closes #3566

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

Fixed an issue where SnippetsFilters were being included in NGINX configuration for all gateways regardless of whether routes attached to those specific gateways actually referenced the filters.

[codecov]

Codecov Report

Attention: Patch coverage is 84.09091% with 7 lines in your changes missing coverage. Please review.

Project coverage is 86.88%. Comparing base (4ccd74f) to head (d88ba23).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/controller/state/graph/gateway.go	81.57%	4 Missing and 3 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3586   +/-   ##
=======================================
  Coverage   86.87%   86.88%           
=======================================
  Files         127      127           
  Lines       15226    15266   +40     
  Branches       62       62           
=======================================
+ Hits        13228    13264   +36     
- Misses       1847     1848    +1     
- Partials      151      154    +3     

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