Keep network denial tests off remote exec path

[starr-openai]

Why

The latest merged main rust-ci-full run (26134557979) failed these Linux remote shard tests:

• codex-core::all::suite::unified_exec::unified_exec_network_denial_emits_failed_background_end_event in https://github.com/openai/codex/actions/runs/26134557979/job/76868559368
• codex-core::all::suite::unified_exec::unified_exec_short_lived_network_denial_emits_failed_end_event in https://github.com/openai/codex/actions/runs/26134557979/job/76868559385

Both failures occurred before the tests reached the managed network-denial behavior they are meant to assert. The logs show exec_command routed through the remote exec-server path and failing with exec-server rejected request (-32603): No such file or directory (os error 2), after which the tests timed out waiting for ExecCommandEnd.

What Changed

• Updated the shared network-denial test helper in codex-rs/core/tests/suite/unified_exec.rs so these two tests use the normal local test environment instead of opting into the unrelated remote exec-server path.

Verification

• Original repro evidence: rust-ci-full run 26134557979, job links above, both failed with the same remote exec-server startup rejection before timing out.
• Targeted repro workflow evidence: https://github.com/openai/codex/actions/runs/26137097959 reproduced unified_exec_network_denial_emits_failed_background_end_event in https://github.com/openai/codex/actions/runs/26137097959/job/76876104957, and https://github.com/openai/codex/actions/runs/26137094301 reproduced unified_exec_short_lived_network_denial_emits_failed_end_event in https://github.com/openai/codex/actions/runs/26137094301/job/76876222111.
• CI on this PR is green and is the authoritative validation path.