Fix Python toolchain configuration in Bazel MODULE.bazel causing build failures

[mmorel-35]

Fixes a Python toolchain configuration issue in the Bazel bzlmod setup that was causing build failures when emsdk is used as a dependency in other projects.

Problem

The build was failing with the error:

Unable to find package for @@[unknown repo 'python3_12' requested from @@envoy~~core~emsdk (did you mean 'python_3_12'?)]//:defs.bzl

This occurred when building WebAssembly targets that depend on the py_binary rule in //emscripten_toolchain:wasm_binary. The error manifested in projects like envoyproxy/examples when they used emsdk as a Bazel module dependency.

Root Cause

The emsdk's MODULE.bazel was not properly configuring the Python toolchain for bzlmod. The Python extension needed is_default = True and explicit use_repo() statements to properly expose the Python repositories to the build system.

Solution

1. Added proper Python toolchain configuration: Set is_default = True in the Python toolchain configuration
2. Added explicit repository exposure: Used use_repo() to explicitly expose python_3_13 and python_versions repositories

The configuration now follows the proper bzlmod pattern:

python.toolchain(
    is_default = True,
    python_version = "3.13",
)
use_repo(
    python,
    "python_3_13",
    python = "python_versions",
)

With the default toolchain properly configured, the py_binary rule can rely on the default Python toolchain without needing an explicit python_version attribute.