test: adds type=CEL LLM cost in Test_Examples_TokenRateLimit

[mathetake]

Description:

Currently, examples/token_rate_limit has only input/output/total token costs configurations, but there's another option that can be used which is CEL expression:

ai-gateway/api/v1alpha1/api.go

Lines 488 to 508 in a6d1ed4

	// CELExpression is the CEL expression to calculate the cost of the request.
	// The CEL expression must return a signed or unsigned integer. If the
	// return value is negative, it will be error.
	//
	// The expression can use the following variables:
	//
	// * model: the model name extracted from the request content. Type: string.
	// * backend: the backend name in the form of "name.namespace". Type: string.
	// * input_tokens: the number of input tokens. Type: unsigned integer.
	// * output_tokens: the number of output tokens. Type: unsigned integer.
	// * total_tokens: the total number of tokens. Type: unsigned integer.
	//
	// For example, the following expressions are valid:
	//
	// * "model == 'llama' ? input_tokens + output_token * 0.5 : total_tokens"
	// * "backend == 'foo.default' ? input_tokens + output_tokens : total_tokens"
	// * "input_tokens + output_tokens + total_tokens"
	// * "input_tokens * output_tokens"
	//
	// +optional
	CELExpression *string `json:"celExpression,omitempty"`

I would like to showcase the use of CEL expression cost API and add test in
https://github.com/envoyproxy/ai-gateway/blob/main/tests/e2e/token_ratelimit_test.go