[WIP] Add the streaming response to prefix cache

[zetxqx]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR introduces the capability to process streaming responses from the model server and add to the approximate prefix cache. It adds a new parser for streaming responses that can handle both chat and legacy completion formats.

• Implemented a new streaming response parser in pkg/epp/scheduling/types/llmresponse.go.
• Updated pkg/epp/handlers/response.go and pkg/epp/handlers/server.go to handle streaming responses.
• Updated pkg/epp/requestcontrol/director.go to use the parsed LLMResponse.
• Added and updated unit tests for the new functionality.

Which issue(s) this PR fixes:

Fixes #971

Does this PR introduce a user-facing change?:

feat: improve approximate prefix cache by putting the response into account especially benefit for multi-turn usecase.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[zetxqx]

/retest

[k8s-ci-robot]

@zetxqx: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gateway-api-inference-extension-test-e2e-main	8e8df08	link	true	/test pull-gateway-api-inference-extension-test-e2e-main
pull-gateway-api-inference-extension-verify-main	8e8df08	link	true	/test pull-gateway-api-inference-extension-verify-main
pull-gateway-api-inference-extension-test-unit-main	8e8df08	link	true	/test pull-gateway-api-inference-extension-test-unit-main

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