`local_runtime_repo` fails on macOS/Xcode Python with watch_tree() error against missing directory

# 🐞 bug report

### Affected Rule

`local_runtime_repo`

### Is this a regression?

No

### Description

The metadata for `/usr/bin/python3` on macOS will return a path that does not exist (by default):

```
DEBUG: local_runtime_repo.bzl:101:10: {"major": 3, "minor": 9, "micro": 6, "include": "/Library/Python/3.9/include", "implementation_name": "cpython", "base_executable": "/Applications/Xcode-16.4.0.app/Contents/Developer/usr/bin/python3", "LDLIBRARY": "Python3.framework/Versions/3.9/Python3", "LIBDIR": "/Applications/Xcode.app/Contents/Developer/Library/Frameworks/Python3.framework/Versions/3.9/lib", "INSTSONAME": "Python3.framework/Versions/3.9/Python3", "PY3LIBRARY": "", "SHLIB_SUFFIX": ".so"}
```

This is pointing to a stock Python3 on macOS. The reported `include` directory does not exist, and will result in the error:

```
Error in watch_tree: can't call watch_tree() on non-directory /Library/Python/3.9/include
```

## 🔬 Minimal Reproduction

Cribbing heavily off of the local toolchain setup in the docs, on macOS 15.5:

```starlark
python.toolchain(python_version = "3.9")

local_runtime_repo = use_repo_rule(
"@rules_python//python/local_toolchains:repos.bzl",
"local_runtime_repo",
)

local_runtime_toolchains_repo = use_repo_rule(
"@rules_python//python/local_toolchains:repos.bzl",
"local_runtime_toolchains_repo",
)

local_runtime_repo(
name = "local_python3",
interpreter_path = "/usr/bin/python3",
on_failure = "warn"
)

local_runtime_toolchains_repo(
name = "local_toolchains",
runtimes = ["local_python3"],
)

# Step 3: Register the toolchains
register_toolchains("@local_toolchains//:all", dev_dependency = True)
```

## 🔥 Exception or Error

2025-07-01 10:07:16 CDT | : in 

-- | --

  | 2025-07-01 10:07:16 CDT | Repository rule local_runtime_repo defined at:

  | 2025-07-01 10:07:16 CDT | /Users/buildkite/.cache/bazel/a876e18133e23b0f96cd6798b2a722d4/external/rules_python+/python/private/local_runtime_repo.bzl:149:37: in 

  | 2025-07-01 10:07:16 CDT | ERROR: /Users/buildkite/.cache/bazel/a876e18133e23b0f96cd6798b2a722d4/external/rules_python+/python/private/repo_utils.bzl:448:25: An error occurred during the fetch of repository '+_repo_rules+local_python3':

  | 2025-07-01 10:07:16 CDT | Traceback (most recent call last):

  | 2025-07-01 10:07:16 CDT | File "/Users/buildkite/.cache/bazel/a876e18133e23b0f96cd6798b2a722d4/external/rules_python+/python/private/local_runtime_repo.bzl", line 102, column 26, in _local_runtime_repo_impl

  | 2025-07-01 10:07:16 CDT | repo_utils.watch_tree(rctx, rctx.path(info["include"]))

  | 2025-07-01 10:07:16 CDT | File "/Users/buildkite/.cache/bazel/a876e18133e23b0f96cd6798b2a722d4/external/rules_python+/python/private/repo_utils.bzl", line 448, column 25, in _watch_tree

  | 2025-07-01 10:07:16 CDT | mrctx.watch_tree(*args, **kwargs)

  | 2025-07-01 10:07:16 CDT | Error in watch_tree: can't call watch_tree() on non-directory /Library/Python/3.9/include

  | 2025-07-01 10:07:16 CDT | ERROR: no such package '@@+_repo_rules+local_python3//': can't call watch_tree() on non-directory /Library/Python/3.9/include

  | 2025-07-01 10:07:16 CDT | ERROR: /Users/buildkite/.cache/bazel/a876e18133e23b0f96cd6798b2a722d4/external/+_repo_rules2+local_toolchains/BUILD.bazel:6:30: @@+_repo_rules2+local_toolchains//:0001_default_py_cc_toolchain depends on @@+_repo_rules+local_python3//:os in repository @@+_repo_rules+local_python3 which failed to fetch. no such package '@@+_repo_rules+local_python3//': can't call watch_tree() on non-directory /Library/Python/3.9/include

  | 2025-07-01 10:07:16 CDT | ERROR: /Users/buildkite/builds/mbr-buildkite-agent-ios-prod-polite-carp-p6d4-1/slack/ios-pull-request/tools/pipeline_generator/BUILD.bazel:16:10: Analysis failed

  | 2025-07-01 10:07:16 CDT | ERROR: Analysis of target '//tools/pipeline_generator:main' failed; build aborted

## 🌍 Your Environment

**Operating System:** macOS 15.5

**Output of `bazel version`:**

  

Bazelisk version: 1.26.0

INFO: Invocation ID: 6e64a830-cfe4-4daf-bc8e-28424507a9c3

Build label: 8.2.1

Build target: @@//src/main/java/com/google/devtools/build/lib/bazel:BazelServer

Build time: Thu Apr 17 18:31:45 2025 (1744914705)

Build timestamp: 1744914705

Build timestamp as int: 1744914705

  

**Rules_python version:**

  

1.5.0

  

**Anything else relevant?**

This can be worked around by creating the include directory:

```
sudo mkdir -p /Library/Python/3.9/include
```

Bắt đầu trong python/private/local_runtime_repo.bzl, tại lệnh gọi watch_tree khoảng dòng 102, sau đó đọc python/private/repo_utils.bzl quanh _watch_tree. Tái hiện việc thiết lập local_runtime_repo với /usr/bin/python3 trên macOS và xác minh rằng rule không còn thất bại khi đường dẫn include được báo cáo không tồn tại hoặc không phải là một thư mục.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.