Add Bun language support

Implements Bun as a new language option for pre-commit hooks, enabling
hooks to run using the Bun JavaScript runtime and package manager.

- Add bun.py language implementation with binary download/install
- Support system-installed Bun or automatic version download
- Add comprehensive tests including version handling and hook execution
- Register bun in all_languages.py
- Include test repository fixture for integration tests

tests/languages/bun_test.py

@@ -0,0 +1,141 @@
+from __future__ import annotations
+
+import sys
+from unittest import mock
+
+import pytest
+
+import pre_commit.constants as C
+from pre_commit import lang_base
+from pre_commit import parse_shebang
+from pre_commit.languages import bun
+from pre_commit.prefix import Prefix
+from pre_commit.store import _make_local_repo
+from testing.language_helpers import run_language
+
+
+ACTUAL_GET_DEFAULT_VERSION = bun.get_default_version.__wrapped__
+
+
+@pytest.fixture
+def find_exe_mck():
+    with mock.patch.object(parse_shebang, 'find_executable') as mck:
+        yield mck
+
+
+def test_sets_system_when_bun_is_available(find_exe_mck):
+    find_exe_mck.return_value = '/path/to/exe'
+    assert ACTUAL_GET_DEFAULT_VERSION() == 'system'
+
+
+def test_uses_default_when_bun_is_not_available(find_exe_mck):
+    find_exe_mck.return_value = None
+    assert ACTUAL_GET_DEFAULT_VERSION() == C.DEFAULT
+
+
+def _make_hello_world(tmp_path):
+    """Create a simple Node/Bun package for testing."""
+    package_json = '''\
+{
+    "name": "test-bun-hook",
+    "version": "1.0.0",
+    "bin": {"bun-hello": "./bin/bun-hello.js"}
+}
+'''
+    bin_script = '''\
+#!/usr/bin/env node
+console.log('Hello World');
+'''
+
+    tmp_path.joinpath('package.json').write_text(package_json)
+    bin_dir = tmp_path.joinpath('bin')
+    bin_dir.mkdir()
+    bin_dir.joinpath('bun-hello.js').write_text(bin_script)
+
+
+def test_bun_default_version():
+    """Test default version detection."""
+    version = bun.get_default_version()
+    # Should return either 'system' or 'default'
+    assert version in {'system', 'default'}
+
+
+@pytest.mark.skipif(
+    not lang_base.exe_exists('bun'),
+    reason='bun not installed on system',
+)
+def test_bun_hook_system(tmp_path):
+    """Test running a hook with system Bun."""
+    _make_hello_world(tmp_path)
+    ret = run_language(tmp_path, bun, 'bun-hello')
+    assert ret == (0, b'Hello World\n')
+
+
+@pytest.mark.skipif(
+    sys.platform == 'win32',
+    reason='Test may be slow on Windows',
+)
+def test_bun_hook_default_version(tmp_path):
+    """Test running a hook with downloaded Bun (default/latest)."""
+    _make_hello_world(tmp_path)
+    ret = run_language(tmp_path, bun, 'bun-hello', version=C.DEFAULT)
+    assert ret == (0, b'Hello World\n')
+
+
+@pytest.mark.skipif(
+    sys.platform == 'win32',
+    reason='Test may be slow on Windows',
+)
+def test_bun_hook_specific_version(tmp_path):
+    """Test running a hook with specific Bun version."""
+    _make_hello_world(tmp_path)
+    # Use a known stable version
+    ret = run_language(tmp_path, bun, 'bun-hello', version='1.1.42')
+    assert ret == (0, b'Hello World\n')
+
+
+def test_bun_additional_dependencies(tmp_path):
+    """Test installing additional dependencies."""
+    _make_local_repo(str(tmp_path))
+    ret, out = run_language(
+        tmp_path,
+        bun,
+        'bun pm ls -g',
+        deps=('lodash',),
+    )
+    assert b'lodash' in out
+
+
+def test_bun_with_package_json_only(tmp_path):
+    """Test that package.json is required."""
+    # Don't create package.json - just create a Prefix
+    prefix = Prefix(str(tmp_path))
+
+    with pytest.raises(AssertionError):
+        bun.install_environment(prefix, 'system', ())
+
+
+def test_environment_dir():
+    """Test ENVIRONMENT_DIR is set correctly."""
+    assert bun.ENVIRONMENT_DIR == 'bunenv'
+
+
+def test_run_hook_uses_basic():
+    """Test that run_hook is set to basic implementation."""
+    assert bun.run_hook is lang_base.basic_run_hook
+
+
+@pytest.mark.skipif(
+    not lang_base.exe_exists('bun'),
+    reason='bun not installed on system',
+)
+def test_bun_health_check_success(tmp_path):
+    """Test health check with valid environment."""
+    _make_hello_world(tmp_path)
+
+    _make_local_repo(str(tmp_path))
+    prefix = Prefix(str(tmp_path))
+    bun.install_environment(prefix, 'system', ())
+
+    health = bun.health_check(prefix, 'system')
+    assert health is None  # None means healthy