Reuse remote exec-server in core tests (#17837)

## Summary
- reuse a shared remote exec-server for remote-aware codex-core
integration tests within a test binary process
- keep per-test remote cwd creation and cleanup so tests retain
workspace isolation
- leave codex_self_exe, codex_linux_sandbox_exe, cwd_path(), and
workspace_path() behavior unchanged

## Validation
- rustfmt codex-rs/core/tests/common/test_codex.rs
- git diff --check
- CI is running on the updated branch

.github/workflows/rust-ci-full.yml

@@ -664,6 +664,7 @@ jobs:
           export CODEX_TEST_REMOTE_ENV_CONTAINER_NAME=codex-remote-test-env
           source "${GITHUB_WORKSPACE}/scripts/test-remote-env.sh"
           echo "CODEX_TEST_REMOTE_ENV=${CODEX_TEST_REMOTE_ENV}" >> "$GITHUB_ENV"
+          echo "CODEX_TEST_REMOTE_EXEC_SERVER_URL=${CODEX_TEST_REMOTE_EXEC_SERVER_URL}" >> "$GITHUB_ENV"
 
       - name: tests
         id: test