Refactor duplicated get_default_ip() function into common utility module

[coderabbitai]

Problem

The get_default_ip() function is currently duplicated in multiple test files:

• tests/test_hear_cleanup.py (lines 176-200)
• tests/test_hear_server.py

This violates DRY principles and creates maintenance overhead.

Proposed Solution

Refactor the get_default_ip() function into a common location, with suggested targets:

1. tests.context module - Most appropriate for test utility functions
2. multicast.skt module - Alternative location if it fits better with socket utilities

Implementation Notes

• Remove duplicate implementations from individual test files
• Import the centralized function where needed
• Ensure consistent behavior across all usage points
• Update any related documentation

Context

This refactoring was identified during code review in PR #448. The function implements RFC 5737 compliant IP detection using TEST-NET-3 (203.0.113.1) and is well-designed but should not be duplicated.

References

• Original discussion: Patch test coverage chore 441 #448 (comment)
• PR: Patch test coverage chore 441 #448