fix: sanitize proxy env vars to prevent InvalidURL from newlines

[C1-BA-B1-F3]

Summary

Fixes #3303 - InvalidURL error when NO_PROXY environment variable contains newline characters.

Problem

When proxy environment variables (like NO_PROXY) contain newline characters (common in Docker environments, .env files, or shell scripts), creating an OpenAI client raises httpx.InvalidURL.

This occurs because httpx's get_environment_proxies() only splits by comma ,, not by newline \n. When a newline is present in the value, it becomes part of the hostname and triggers URL validation failure.

Reproduction

import os
os.environ['NO_PROXY'] = 'localhost\n192.168.1.1'

from openai import OpenAI
client = OpenAI(api_key='sk-test')
# Raises: httpx.InvalidURL: Invalid non-printable ASCII character in URL, '\n' at position 16

Solution

Added sanitize_proxy_env_vars() utility function that:

• Replaces newlines (\n, \r) with commas in proxy environment variables
• Strips whitespace from each entry
• Removes empty entries
• Is called in _DefaultHttpxClient.__init__, _DefaultAsyncHttpxClient.__init__, and _DefaultAioHttpClient.__init__ before httpx reads them

Changes

• src/openai/_utils/_utils.py: Added sanitize_proxy_env_vars() function
• src/openai/_utils/__init__.py: Exported the new function
• src/openai/_base_client.py: Call sanitization in default httpx client classes
• tests/test_utils/test_sanitize_proxy_env.py: Added comprehensive tests

Testing

All tests pass, including integration test verifying that creating _DefaultHttpxClient with newlines in NO_PROXY no longer raises.

Fixes #3303