[receiver/statsd] Clean up stale unix socket on startup

[Vanshul97]

Summary

Clean up stale unix socket files on startup to prevent "address already in use" errors when the StatsD receiver restarts after an unclean shutdown.

Fixes #44866

Problem

When the StatsD receiver is configured with UDS (unix domain socket) transport and the collector process is killed ungracefully (SIGKILL, OOM, crash), the socket file is not cleaned up. On restart, net.ListenPacket fails because the stale socket file still exists:

starting to listen unixgram socket: listen unixgram /var/run/statsd.sock: bind: address already in use

The Close() method does remove the socket file, but it never runs during an unclean shutdown.

Fix

Added os.Remove(socketPath) before net.ListenPacket() in NewUDSServer(). This removes any stale socket file from a previous instance. The error is intentionally ignored since the file won't exist on first startup.

This is the standard pattern for unix socket servers, used throughout the Go ecosystem and within this repo (e.g., internal/docker/docker_test_listener.go, receiver/podmanreceiver).

Test Plan

• Added Test_NewUDSServer_CleansUpStaleSocket that simulates a crash (closes connection without removing socket file) and verifies a new server can start on the same path
• Code compiles cleanly
• Existing tests unaffected (linux-only build constraint)

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