Stabilize exec-server filesystem tests in CI (#17671)

## Summary\n- add an exec-server package-local test helper binary that
can run exec-server and fs-helper flows\n- route exec-server filesystem
tests through that helper instead of cross-crate codex helper
binaries\n- stop relying on Bazel-only extra binary wiring for these
tests\n\n## Testing\n- not run (per repo guidance for codex changes)

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/test-binary-support/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "codex-test-binary-support"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[lib]
+path = "lib.rs"
+
+[lints]
+workspace = true
+
+[dependencies]
+codex-arg0 = { workspace = true }
+tempfile = { workspace = true }