[codex] Test code-mode variable truncation

[aibrahim-oai]

Summary

Code mode has two separate truncation points: the nested tool result returned to JavaScript and the code-mode output later recorded for the model. These tests now verify those behaviors independently.

• Report whether result.output was truncated before printing it.
• Verify omitted or sufficiently large nested limits produce Variable truncated: False, while allowing the printed value to be truncated downstream.
• Verify an explicit nested limit produces Variable truncated: True when the command output exceeds it.
• Use a token-policy model fixture so downstream truncation is visible as …N tokens truncated….
• Align the explicit nested-truncation expectation with the warning header.

This PR changes test coverage only; runtime truncation behavior is unchanged.

Validation

• env -u CODEX_SANDBOX_NETWORK_DISABLED RUST_MIN_STACK=8388608 cargo test -p codex-core --test all code_mode_exec -- --nocapture (8 passed)

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Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
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Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".