Fix python_server.py infinite loop on EOF (fixes #25620)

[anthonykim1]

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():

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

# 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