[Feature]: Add lock-based thread-safe IPv4/IPv6 transport adapters #15

Describe the feature you want to add

The current IPv4TransportAdapter and IPv6TransportAdapter are not thread-safe. They temporarily replace the global socket.getaddrinfo function during each send() call without any synchronization. In multi-threaded applications (for example using ThreadPoolExecutor or threaded web servers), concurrent requests can race to patch/restore getaddrinfo, leading to corrupted DNS resolution, leaked patches, or incorrect address family filtering.

We need a thread-safe adapter variant that guarantees correctness under concurrent access. The simplest and most universally compatible approach is a process-wide threading.Lock that serializes access to the socket.getaddrinfo patch. This ensures that only one thread at a time can modify and read the global function, eliminating all race conditions.

Describe your proposed solution

Introduce IPv4LockAdapter and IPv6LockAdapter classes that extend HTTPAdapter. They share a module-level threading.Lock. Before patching socket.getaddrinfo, the adapter acquires the lock; after the underlying send() completes (or raises), the lock is released and getaddrinfo is restored.

Key design choices:

• Single shared lock across all lock-based adapter instances (both IPv4 and IPv6)
• Compatible with Python 3.7+ (no new stdlib dependencies beyond threading)
• Trade-off: DNS resolution is serialized across threads (throughput limited)

Describe alternatives you've considered, if relevant

No response

Additional context

No response