Update SnippetFilter validation rules in enhancement proposal

[kate-osborn]

Proposed changes

Problem: The advanced nginx extensions proposal doesn't provide guidance on handling routes that reference invalid or nonexistent SnippetFilters.

Solution: Add guidance on handling these cases to the enhancement proposal. The guidance is based off the Gateway API spec documentation.

Closes #2468

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.

None

[bjee19]

@kate-osborn the link to the closed issue points to a closed PR instead of #2468.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.79%. Comparing base (1e65743) to head (c7219fa).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2471   +/-   ##
=======================================
  Coverage   88.79%   88.79%           
=======================================
  Files         100      100           
  Lines        7531     7531           
  Branches       50       50           
=======================================
  Hits         6687     6687           
  Misses        788      788           
  Partials       56       56           

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