[Fix] nvm_download, nvm_get_latest, install: only select a downloader that exists as an executable

`nvm_has` matches shell functions and aliases,
but downloads now run via `command`, which skips them
- a `curl` shell function with no curl binary on the PATH would select the curl path and fail with exit 127,
instead of falling back to an available wget executable.
The new `nvm_has_executable` helper resolves names the same way `command` does, so downloader selection and execution agree.

test/fast/Unit tests/nvm install --offline

@@ -30,9 +30,10 @@ nvm_echo "$CAPTURED_STDERR" | nvm_grep -q "${EXPECTED_ERR}" \
 
 # --offline should not require curl or wget
 nvm_has() { return 1; }
+nvm_has_executable() { return 1; }
 try_err nvm install --offline 999.999.999
 # Should fail with "not found" not "nvm needs curl or wget"
 nvm_echo "$CAPTURED_STDERR" | nvm_grep -q "curl or wget" \
   && die "nvm install --offline should not require curl or wget"
 alias nvm_has='\nvm_has'
-unset -f nvm_has
+unset -f nvm_has nvm_has_executable

test/fast/Unit tests/nvm_download and curl helpers bypass shadowed curl

@@ -45,4 +45,22 @@ nvm_curl_libz_support || die 'nvm_curl_libz_support used the shadowing curl func
 nvm_download -s 'http://example.com/' -o /dev/null || die 'nvm_download used the shadowing curl function and failed'
 grep -Fxq 'http://example.com/' "$ARGV_LOG" || die "nvm_download did not invoke the curl executable; got: $(cat "$ARGV_LOG")"
 
+# when no curl executable exists, a shadowing curl shell function must not fool
+# downloader selection: nvm_download must fall back to the wget executable
+WGET_ONLY_BIN="$WORK/wget-only-bin"
+mkdir -p "$WGET_ONLY_BIN"
+{
+  echo '#!/bin/sh'
+  echo ': > "$ARGV_LOG"'
+  echo 'for a in "$@"; do printf "%s\n" "$a" >> "$ARGV_LOG"; done'
+} > "$WGET_ONLY_BIN/wget"
+chmod +x "$WGET_ONLY_BIN/wget"
+
+(
+  PATH="$WGET_ONLY_BIN"
+  export PATH
+  nvm_download -s 'http://wget-fallback.example.com/' -o /dev/null
+) || die 'nvm_download did not fall back to wget when curl is only a shell function'
+grep -Fxq 'http://wget-fallback.example.com/' "$ARGV_LOG" || die "nvm_download did not invoke the wget executable; got: $(cat "$ARGV_LOG")"
+
 cleanup

test/fast/Unit tests/nvm_download wget Authorization header

@@ -6,7 +6,7 @@ TEST_BIN="$WORK/bin"
 ARGV_LOG="$WORK/argv.log"
 
 cleanup() {
-  unset -f die cleanup nvm_has
+  unset -f die cleanup nvm_has_executable
   rm -rf "$WORK"
   export PATH="$OLDPATH"
 }
@@ -30,7 +30,7 @@ chmod +x "$TEST_BIN/wget"
 export ARGV_LOG
 export PATH="$TEST_BIN:$OLDPATH"
 # force the wget path while keeping system tools (sed) available for sanitization
-nvm_has() { [ "$1" != curl ] && command -v "$1" >/dev/null 2>&1; }
+nvm_has_executable() { [ "$1" != curl ] && command -v "$1" >/dev/null 2>&1; }
 
 # given an Authorization credential in NVM_AUTH_HEADER
 # when nvm_download uses the wget path

test/fast/Unit tests/nvm_get_latest missing curl or wget

@@ -3,7 +3,7 @@
 die () { echo "$@" ; cleanup ; exit 1; }
 
 cleanup() {
-  unset -f nvm_has
+  unset -f nvm_has nvm_has_executable
 }
 
 : nvm.sh
@@ -12,6 +12,7 @@ cleanup() {
 \. ../../common.sh
 
 nvm_has() { return 1 ; }
+nvm_has_executable() { return 1 ; }
 
 try_err nvm_get_latest
 [ "_$CAPTURED_STDERR" = "_nvm needs curl or wget to proceed." ] \