use unix socket to share fd

[Binyang2014]

Use unix socket to share fd to other processes. Used for nvls handle sharing

[Copilot]

Pull Request Overview

This PR replaces the pidfd mechanism for file descriptor sharing with a Unix socket-based approach. The change eliminates dependencies on Linux-specific system calls (SYS_pidfd_open and SYS_pidfd_getfd) in favor of a more portable Unix domain socket implementation.

• Introduces UnixSocketServer and UnixSocketClient classes for file descriptor transmission
• Updates RegisteredMemory and NvlsConnection to use Unix sockets instead of pidfd
• Integrates Unix socket server lifecycle with TcpBootstrap initialization

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/unix_socket.cc	Implements Unix socket server/client for file descriptor transfer
src/include/unix_socket.hpp	Defines UnixSocketServer and UnixSocketClient class interfaces
src/registered_memory.cc	Replaces pidfd usage with Unix socket client calls
src/nvls.cc	Updates NVLS connection to use Unix socket for file descriptor sharing
src/include/registered_memory.hpp	Changes TransportInfo structure from pid/fd to fdId/localRankId
src/bootstrap/bootstrap.cc	Integrates Unix socket server startup and shutdown with bootstrap
include/mscclpp/env.hpp	Adds localRank field to Env class

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[Binyang2014]

/azp run

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Azure Pipelines successfully started running 3 pipeline(s).

[Binyang2014]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[Binyang2014]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[caiomcbr]

LGTM