fix: add validation for request buffer limit

[kkk777-7]

What this PR does / why we need it:
Added gatewayapi layer validation for request buffer limit in BackendTrafficPolicy.

Which issue(s) this PR fixes:

Fixes #7677

Release Notes: Yes

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 72.41%. Comparing base (aa3ad43) to head (00162ed).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/gatewayapi/backendtrafficpolicy.go	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7687      +/-   ##
==========================================
+ Coverage   72.36%   72.41%   +0.05%     
==========================================
  Files         233      233              
  Lines       34343    34347       +4     
==========================================
+ Hits        24851    24873      +22     
+ Misses       7712     7699      -13     
+ Partials     1780     1775       -5     

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[zirain]

is it possible to validate this with CEL?

[kkk777-7]

is it possible to validate this with CEL?

I've thought about this a bit, but given the possible input types, it seems somewhat difficult to implement strict validation at the CRD layer.
https://github.com/envoyproxy/gateway/blob/main/api/v1alpha1/backendtrafficpolicy_types.go#L181-L183

I feel it’s best to limit the CRD validation to format checks and handle min/max constraints within the application code instead. What do you think?