Stabilize exec-server filesystem tests in CI

Move the arg0-based test-binary dispatch bootstrap into a shared test-only crate so core and exec-server can reuse the same current_exe-based harness without keeping exec-server-specific helper logic in codex-arg0.

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

codex-rs/test-binary-support/BUILD.bazel

@@ -0,0 +1,7 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "test-binary-support",
+    crate_name = "codex_test_binary_support",
+    crate_srcs = ["lib.rs"],
+)