[Python] Tests fail on Windows with spaces in username due to improper URI encoding

### Describe the bug, including details regarding any error messages, version, and platform.

### Describe the bug

I'm running PyArrow tests on Windows with a username that contains a space ("Eslam Ahmed"), and I'm encountering **9 test failures**. Upon investigation, **8 failures** are caused by the `_filesystem_uri()` helper function in `pyarrow/tests/util.py`, and **1 failure** is caused by similar manual URI construction in a test file.

**Example error:**
```
pyarrow.lib.ArrowInvalid: Cannot parse URI: 'file:///C:/Users/Eslam Ahmed/AppData/Local/Temp/pytest-of-Eslam Ahmed/pytest-2/test_filesystem_uri0'
due to syntax error at character ' ' (position 22)
```

### Root Cause

The issue is in the `_filesystem_uri()` function in `pyarrow/tests/util.py` which doesn't URL-encode spaces:

**Current problematic code:**
```python
def _filesystem_uri(path):
# URIs on Windows must follow 'file:///C:...' or 'file:/C:...' patterns.
if os.name == 'nt':
uri = f'file:///{path}'
else:
uri = f'file://{path}'
return uri
```

This creates URIs like `file:///C:/Users/Eslam Ahmed/test` instead of `file:///C:/Users/Eslam%20Ahmed/test`, violating RFC 3986 which requires spaces to be percent-encoded.

### Impact

This affects:
1. Users with spaces in their Windows usernames
2. Multiple test failures (8+ tests fail when username contains spaces)
3. Any code using `_filesystem_uri()` with paths containing special characters

### Proposed Solution

Use Python's built-in `Path.as_uri()` which handles proper percent-encoding:

**For `_filesystem_uri()` in `pyarrow/tests/util.py`:**
```python
from pathlib import Path

def _filesystem_uri(path):
return Path(path).as_uri()
```

This automatically handles:
- Proper percent-encoding of spaces and special characters
- Correct URI scheme for both Windows and Unix
- Cross-platform compatibility

**For the manual URI construction in `pyarrow/tests/parquet/test_metadata.py` (line ~690):**
```python
# From:
file_uri = 'file:///' + file_path

# To:
from pathlib import Path
file_uri = Path(file_path).as_uri()
```

### Environment

- **OS**: Windows 11
- **PyArrow version**: main branch (development build)
- **Python version**: 3.13

I'm happy to submit a PR with the fix if this approach looks good to the maintainers.

### Component(s)

Python