## Summary

Fixes #25620 - Leftover process `python_server.py` with 100% CPU after
closing VS Code.

## The Problem

When VS Code closes, the STDIN stream to `python_server.py` is closed.
The `readline()` method returns empty bytes (`b''`) to signal EOF.
However, the previous code incorrectly treated this as an empty line
separator, causing:

1. `get_headers()` to return an empty headers dict
2. `content_length` to be 0
3. The main loop to continue immediately back to `get_headers()`
4. An infinite loop consuming 100% CPU

## The Fix

This PR properly detects EOF by checking for `b''` (empty bytes) vs
`b'\r\n'` or `b'\n'` (actual empty line with newline characters):

**In `get_headers()`:**
```python
raw = STDIN.buffer.readline()
# Detect EOF: readline() returns empty bytes when input stream is closed
if raw == b"":
    raise EOFError("EOF reached while reading headers")
```

**In all callers (main loop, `handle_response()`, `custom_input()`):**
- Catch `EOFError` and exit gracefully with `sys.exit(0)`

## Key Insight

```python
# EOF: readline() returns empty bytes
io.BytesIO(b"").readline()  # Returns b''

# Empty line: readline() returns newline bytes
io.BytesIO(b"\r\n").readline()  # Returns b'\r\n'
```

## Testing

Added comprehensive unit tests in
`python_files/tests/test_python_server.py`:
- `test_get_headers_normal` - verifies normal header parsing still works
- `test_get_headers_eof_raises_error` - verifies EOF detection
- `test_get_headers_eof_mid_headers_raises_error` - EOF during header
reading
- `test_get_headers_empty_line_terminates` - empty line still terminates
headers
- `test_custom_input_exits_on_eof` - graceful exit from `custom_input()`
- `test_handle_response_exits_on_eof` - graceful exit from
`handle_response()`
- `test_main_loop_exits_on_eof` - simulates the main loop behavior
- `test_readline_eof_vs_empty_line` - documents the EOF vs empty line
distinction

All 8 tests pass.

## How to Verify

1. Open a Python file in VS Code
2. Use Shift+Enter to start a Native REPL session and run some commands
3. Close VS Code
4. Check for leftover processes: `ps aux | grep python_server`
5. With this fix, no leftover processes should remain