[responsesAPI][ez] add a unit test for SimpleContext logprobs

[qandrew]

Purpose

this adds a unit test to make sure logprobs are outputted correctly, as it would look like this in streaming. no behavioral changes. Adding so I can be more confident in the migration in #37007.

event: response.output_text.delta
data: {"content_index":0,"delta":" 😊","item_id":"090bc9aa-4861-4e04-b2cc-aeb05bb9ab03","logprobs":[{"token":"","logprob":-2.622600959512056e-6,"top_logprobs":[{"token":"","logprob":-2.622600959512056e-6},{"token":"","logprob":-12.87500286102295},{"token":"","logprob":-17.625001907348633}]}],"output_index":1,"sequence_number":138,"type":"response.output_text.delta"}

event: response.output_text.delta
data: {"content_index":0,"delta":" How","item_id":"090bc9aa-4861-4e04-b2cc-aeb05bb9ab03","logprobs":[{"token":" How","logprob":0.0,"top_logprobs":[{"token":" How","logprob":0.0},{"token":"  \n","logprob":-17.0},{"token":" I","logprob":-18.375}]}],"output_index":1,"sequence_number":139,"type":"response.output_text.delta"}

event: response.output_text.delta
data: {"content_index":0,"delta":" can","item_id":"090bc9aa-4861-4e04-b2cc-aeb05bb9ab03","logprobs":[{"token":" can","logprob":0.0,"top_logprobs":[{"token":" can","logprob":0.0},{"token":" I","logprob":-18.5},{"token":" Can","logprob":-19.875}]}],"output_index":1,"sequence_number":140,"type":"response.output_text.delta"}

event: response.output_text.delta
data: {"content_index":0,"delta":" I","item_id":"090bc9aa-4861-4e04-b2cc-aeb05bb9ab03","logprobs":[{"token":" I","logprob":0.0,"top_logprobs":[{"token":" I","logprob":0.0},{"token":" assist","logprob":-18.875},{"token":"我","logprob":-21.125}]}],"output_index":1,"sequence_number":141,"type":"response.output_text.delta"}

Test Plan

unit test passes
python -m pytest tests/entrypoints/openai/responses/test_simple.py::test_streaming_logprobs

[gemini-code-assist]

Code Review

This pull request adds a unit test for streaming logprobs in the responses API. The test verifies that when logprobs are requested, the streaming events contain the correct logprob data structure. My review focuses on strengthening this new test to ensure it fully validates the expected behavior. I've suggested an improvement to assert that top_logprobs are not just present but also contain data, which makes the test more robust.

[qandrew]

cc @chaunceyjiang / @houseroad could I get a quick review here?

[houseroad]

Looks good.