test: generalize exec-server fixture

[anp-oai]

Why

Remote-executor integration tests need one host-agnostic exec-server fixture target instead of a Windows-only wrapper.

What

• rename the testing binary target to exec-server
• make the fixture source and target host-agnostic
• update Windows remote-executor test wiring to use the shared target

Validation

• bazel build //codex-rs/exec-server/testing:exec-server
• bazel cquery --config=ci-windows-cross 'set(//codex-rs/exec-server/testing:exec-server //codex-rs/core/tests/remote_env_windows:smoke-test)'

Stack

1. #31422 test: generalize exec-server fixture
2. #31425 test: add TestAppServer builder
3. #31427 test: add delayed exec-server transport
4. #31295 bench: add cold skill load macrobenchmark
5. #31428 bench: add e2e benchmark entrypoints
6. #31429 ci: smoke Bazel e2e benchmarks

[chatgpt-codex-connector]

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