Expect Codex's plain-text form of a text-only tool result in the native function stream test

[SaladDay]

Summary

This is a test-only fix. TestNativePublicFunctionStreamHelper expected the model to receive the text-only setup tool result as a one-item array. Pinned Codex 0.153.4 deliberately turns a result made of exactly one input_text item into plain text (function_tool_response in codex-rs/core/src/tools/context.rs, asserted by Codex's own function_payloads_remain_function_outputs). Both forms are valid Responses input: the pinned SDK types function_call_output.output as a string or a list. Public history still keeps the array, and the test's SDK script checks that. The fake model server now expects the string, with a one-line comment explaining why.

Verification

• Native run with the daemon built from the branch, pinned Codex 0.153.4 and the official SDK. It failed before the fix ("complete result changed: setup complete") and passes after it in 5.4 s. The other three native tests that use the fake model server also pass.
• Full local server gate on dc7f5663: passed. The Playwright browser cases were skipped on the server (Chrome unavailable; approved skip). Main has since moved only by Show the command that connects a self-hosted executor host #143, a Web-only change that doesn't touch this file.
• Review: self-review, because the change is test-only (2 lines).

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