add missing unit-tests in envoy util functions

[nirrozenbaum]

recently we've added envoy util functions that are missing unit-tests.
some examples:

• gateway-api-inference-extension/pkg/common/envoy/test/utils.go

  Line 27 in 43594ee

  func SortSetHeadersInResponses(responses []*extProcPb.ProcessingResponse) {

• gateway-api-inference-extension/pkg/common/envoy/request/headers.go

  Line 49 in 43594ee

  func GenerateHeadersMutation(headers map[string]string) []*corev3.HeaderValueOption {

we need to make sure we test all util functions to validate correctness.

[yehuditkerido]

/assign