fix(responses): honor include on input item retrieval.

[skamenan7]

What does this PR do?

Closes #5345

The GET /responses/{id}/input_items endpoint accepted the include query parameter but raised NotImplementedError when it was actually set. This wires up the filtering logic so gated fields (file_search_call.results, message.input_image.image_url, message.output_text.logprobs) are nulled out by default and only populated when the caller explicitly requests them via include, matching the OpenAI whitelist contract.

Include values whose backing models don't exist yet in Llama Stack (web_search_call.action.sources, reasoning.encrypted_content, computer_call_output.output.image_url, code_interpreter_call.outputs) are accepted silently as no-ops.

Changes:

• src/llama_stack/providers/utils/responses/responses_store.py -- added _apply_include_filter() and _filter_message_include_fields() private functions, removed the NotImplementedError guard, and called the filter after pagination in list_response_input_items()
• tests/unit/utils/responses/test_responses_store.py -- added 5 new tests covering all include filter branches plus a store round-trip test that verifies stored data is never mutated by filtering
• tests/unit/providers/responses/builtin/test_openai_responses_core.py -- updated the delegation test to use a real ResponseItemInclude enum value instead of a bare string

Test Plan

uv run pytest tests/unit/utils/responses/test_responses_store.py -v --timeout=60

Also,

• Ran focused live integration coverage for the Responses input-items listing path against a local minimal Llama Stack + Ollama setup.
• Verified the new GET /v1/responses/{response_id}/input_items include behavior end to end using a deterministic stored response fixture.
• Confirmed file_search_call.results is hidden by default and only returned when include=file_search_call.results is requested.
• Confirmed message.output_text.logprobs is hidden by default and only returned when
  include=message.output_text.logprobs is requested.
• Confirmed both include values work together, and that an unrelated no-op include value does not break the route.