fix(windows CI): docker_vendor_common_selftest ran against the WSL bash stub

has_bash() only checked that `bash` SPAWNED — on windows-latest,
Command::new("bash") resolves to the System32 WSL stub (no distro
installed), which spawns fine but cannot run a script: it prints its
error as UTF-16 on stdout and exits non-zero, failing all three
stage_patch selftests with empty stderr. Probe an actual `bash -c` run
and require the sentinel back, so hosts without a functional bash skip
(the snippets' real target is the Linux docker images; unix CI hosts
keep full coverage).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: mikolalysenko

crates/socket-patch-cli/tests/docker_vendor_common_selftest.rs

@@ -25,13 +25,19 @@ use docker_vendor_common::{bash_prelude, stage_patch_fn};
 const SHA256SUM_SHIM: &str =
     "command -v sha256sum >/dev/null 2>&1 || sha256sum() { shasum -a 256 \"$@\"; }\n";
 
+/// A *functional* bash, not merely a spawnable one: on windows-latest
+/// `Command::new("bash")` resolves to the System32 WSL stub, which spawns
+/// fine with no distro installed but cannot run a script (it prints its
+/// error as UTF-16 on stdout and exits non-zero) — `.status().is_ok()`
+/// passed on it and the suite ran against a bash that executes nothing.
+/// Probe an actual `-c` run and require the sentinel to come back.
 fn has_bash() -> bool {
     Command::new("bash")
-        .arg("--version")
-        .stdout(Stdio::null())
+        .args(["-c", "echo bash-probe-ok"])
         .stderr(Stdio::null())
-        .status()
-        .is_ok()
+        .output()
+        .map(|o| o.status.success() && String::from_utf8_lossy(&o.stdout).contains("bash-probe-ok"))
+        .unwrap_or(false)
 }
 
 /// Run `body` under the same prelude + stage_patch definitions the docker