`_make_pyi_files` in `reflex/custom_components/custom_components.py` generates the
`.pyi` stubs for a custom component by walking its source tree. That loop used
`pathlib.Path.walk()`, which was only added in Python 3.12. Since the project
supports `requires-python = ">=3.10,<4.0"`, every custom-component author on 3.10
or 3.11 hits a hard crash when building their component.

## Why / root cause

`Path.walk()` does not exist before Python 3.12, and there is no version guard or
fallback around the call. The sibling helper `_walk_files` in
`packages/reflex-base/src/reflex_base/utils/pyi_generator.py` already avoids
`Path.walk()` for exactly this reason (its comment notes it can only be used once
the floor is 3.12), so this second call site had regressed relative to the rest
of the codebase.

## Fix

Switch the walk to `os.walk()` (`os` is already imported in the module) and read
the directory basename via `Path(dir).name` for the `__pycache__` skip.
`PyiGenerator.scan_all` normalizes its targets with `Path(...)`, so the directory
strings that `os.walk` yields work unchanged. This mirrors the existing
`_walk_files` approach and keeps the behavior identical on 3.12+.

## Tests

Added `tests/units/custom_components/test_custom_components.py`. It deletes
`Path.walk` via `monkeypatch` to reproduce the 3.10/3.11 environment on any
interpreter (so the guard also holds on 3.12+, where a naive test would silently
pass), then asserts `_make_pyi_files` recurses into component directories,
skips `__pycache__`, and skips top-level dotdirs, without raising.

- Fails before the fix with the reported `AttributeError`; passes after.
- `uv run pytest tests/units` -> 6617 passed, 18 skipped (no regressions).
- `uv run ruff check .` / `uv run ruff format --check` clean;
  `uv run pyright reflex tests` -> 0 errors.