[Robustness] nvm install: reject a structurally broken installed version

`nvm_is_version_installed` only checks that `bin/node` has the execute bit,
which a zero-byte binary and a dangling `npm` symlink both pass, so a partial
install could be reported as a success — and a broken existing version could
short-circuit `nvm install` as "already installed".

Add `nvm_validate_install`, which requires a non-empty `bin/node` and an `npm`
entry that resolves, and use it in two places: gate the "already installed"
shortcut on it, so a broken version is reinstalled rather than reused, and
re-check it after an install reports success, so a broken result fails loudly
instead of being activated.

It checks layout, not execution: a correctly installed binary can still fail
to run on an incompatible host (e.g. a newer node on an older glibc), which is
not a broken install, and a corrupt download is already rejected by the
checksum check before extraction.

test/fast/Unit tests/nvm install verifies the installed version

@@ -0,0 +1,71 @@
+#!/bin/sh
+
+cleanup() {
+  [ -n "${NVM_DIR}" ] && [ -d "${NVM_DIR}" ] && rm -rf "${NVM_DIR}"
+  unset -f die cleanup make_healthy make_broken \
+    nvm_remote_version nvm_has_executable nvm_binary_available \
+    nvm_install_binary nvm_install_source nvm_use_if_needed \
+    nvm_install_npm_if_needed nvm_install_default_packages nvm_ensure_default_set
+  unset NVM_DIR version version_path REINSTALL_MARKER OUTPUT EXIT_CODE
+}
+
+die() { echo "$@" ; cleanup ; exit 1; }
+
+: nvm.sh
+\. ../../../nvm.sh
+
+unset npm_config_prefix NPM_CONFIG_PREFIX
+
+NVM_DIR="$(mktemp -d)"
+[ -n "${NVM_DIR}" ] || die 'unable to create temp NVM_DIR'
+
+version='v20.0.0'
+version_path="${NVM_DIR}/versions/node/${version}"
+REINSTALL_MARKER="${NVM_DIR}/reinstalled"
+
+# Stay offline and keep the post-install activation path inert; only the
+# verification behaviour is under test here.
+nvm_remote_version() { nvm_echo "${version}"; }
+nvm_has_executable() { return 0; }
+nvm_binary_available() { return 0; }
+nvm_install_source() { return 1; }
+nvm_use_if_needed() { return 0; }
+nvm_install_npm_if_needed() { return 0; }
+nvm_install_default_packages() { return 0; }
+nvm_ensure_default_set() { return 0; }
+
+make_healthy() {
+  mkdir -p "${version_path}/bin"
+  printf '#!/bin/sh\necho "%s"\n' "${version}" > "${version_path}/bin/node"
+  chmod +x "${version_path}/bin/node"
+}
+make_broken() {
+  mkdir -p "${version_path}/bin"
+  : > "${version_path}/bin/node" # zero-byte but +x: passes nvm_is_version_installed
+  chmod +x "${version_path}/bin/node"
+}
+
+# 1) An install that reports success but leaves a broken node must fail loudly
+#    and must not be activated.
+rm -rf "${version_path}"
+nvm_install_binary() { make_broken; return 0; }
+OUTPUT="$(nvm install -b "${version}" 2>&1)"; EXIT_CODE=$?
+[ "${EXIT_CODE}" != "0" ] || die "a broken install should fail; got exit 0, output >${OUTPUT}<"
+case "${OUTPUT}" in
+  *'failed verification'*) ;;
+  *) die "expected a verification-failure message; got >${OUTPUT}<" ;;
+esac
+
+# 2) An already-present but broken version must not short-circuit as "already
+#    installed"; it must be reinstalled.
+rm -rf "${version_path}"
+make_broken
+nvm_install_binary() { command touch "${REINSTALL_MARKER}"; make_healthy; return 0; }
+OUTPUT="$(nvm install -b "${version}" 2>&1)"; EXIT_CODE=$?
+case "${OUTPUT}" in
+  *'is already installed'*) die "a broken version should not be treated as already installed: >${OUTPUT}<" ;;
+esac
+[ -f "${REINSTALL_MARKER}" ] || die 'a broken already-present version was not reinstalled'
+[ "${EXIT_CODE}" = "0" ] || die "the healing reinstall should succeed; got ${EXIT_CODE}, output >${OUTPUT}<"
+
+cleanup

test/fast/Unit tests/nvm_validate_install

@@ -0,0 +1,53 @@
+#!/bin/sh
+
+cleanup() {
+  [ -n "${NVM_DIR}" ] && [ -d "${NVM_DIR}" ] && rm -rf "${NVM_DIR}"
+  unset -f die cleanup
+  unset NVM_DIR version version_path
+}
+
+die() { echo "$@" ; cleanup ; exit 1; }
+
+: nvm.sh
+\. ../../../nvm.sh
+
+type nvm_validate_install > /dev/null 2>&1 || die 'nvm_validate_install is not available'
+
+NVM_DIR="$(mktemp -d)"
+[ -n "${NVM_DIR}" ] || die 'Unable to create temporary folder'
+
+version='v20.0.0'
+version_path="${NVM_DIR}/versions/node/${version}"
+
+# No version directory at all: not valid.
+nvm_validate_install "${version}" 2>/dev/null && die 'a missing version dir should not validate'
+
+# A healthy install: a non-empty node plus an npm symlink that resolves.
+# (validation checks layout, not execution, so bin/node need not be a real
+# runnable binary here - just non-empty and executable.)
+mkdir -p "${version_path}/bin" "${version_path}/lib/node_modules/npm/bin" || die 'setup mkdir failed'
+printf '#!/bin/sh\necho "%s"\n' "${version}" > "${version_path}/bin/node"
+chmod +x "${version_path}/bin/node"
+echo 'npm-cli' > "${version_path}/lib/node_modules/npm/bin/npm-cli.js"
+( cd "${version_path}/bin" && ln -s ../lib/node_modules/npm/bin/npm-cli.js npm )
+nvm_validate_install "${version}" 2>/dev/null || die 'a healthy install should validate'
+
+# A binary that is correctly installed but cannot run on this host (e.g. a
+# newer node on an older glibc) must STILL validate: whether it runs is the
+# host's concern, not a broken install.
+printf '#!/bin/sh\nexit 1\n' > "${version_path}/bin/node"
+chmod +x "${version_path}/bin/node"
+nvm_validate_install "${version}" 2>/dev/null || die 'a non-runnable but present binary should still validate'
+
+# A zero-byte but executable node: passes `[ -x ]` but must not validate.
+: > "${version_path}/bin/node"
+chmod +x "${version_path}/bin/node"
+nvm_validate_install "${version}" 2>/dev/null && die 'a zero-byte node should not validate'
+
+# A non-empty node again, but with a dangling npm symlink: must not validate.
+printf '#!/bin/sh\necho "%s"\n' "${version}" > "${version_path}/bin/node"
+chmod +x "${version_path}/bin/node"
+rm -f "${version_path}/lib/node_modules/npm/bin/npm-cli.js"
+nvm_validate_install "${version}" 2>/dev/null && die 'a dangling npm symlink should not validate'
+
+cleanup