tools: create print_defined_toolchains.sh utility script

 - Added shell script under tools/private/toolchains to query all toolchain rules inside the Bzlmod pythons_hub repository.

 - Enabled executable permission on the script.

 - Updated bazel query to filter specifically for '//python:toolchain_type' attribute, excluding CC and Exec Tools toolchains.

Author: rickeylev

tools/private/debug/print_defined_toolchains.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Programmatically probe which repository target name is resolved successfully inside this workspace
+if bazel query @pythons_hub//... >/dev/null 2>&1; then
+  HUB_REPO="@pythons_hub"
+elif bazel query @@rules_python++python+pythons_hub//... >/dev/null 2>&1; then
+  HUB_REPO="@@rules_python++python+pythons_hub"
+else
+  HUB_REPO="@@+python+pythons_hub"
+fi
+
+# Query standard toolchains inside the resolved hub repository, excluding CC and Exec Tools toolchains.
+bazel query "kind('toolchain', ${HUB_REPO}//...) - filter('_py_cc_toolchain$', ${HUB_REPO}//...) - filter('_py_exec_tools_toolchain$', ${HUB_REPO}//...)" "$@"