fix: set socket buffer options before connect

[howzi]

What problem does this PR solve?

现有的tcp SNDBUF和RCVBUF设置时机有问题，不仅会导致设置失败，测试中发现还会导致buf固定位非预期的值。

参考:
https://man7.org/linux/man-pages/man7/tcp.7.html

[Copilot]

Pull request overview

This PR fixes the timing of applying TCP socket buffer size options (SO_SNDBUF/SO_RCVBUF) so they take effect reliably by setting them before connect() on client sockets and before bind()/listen() on server listening sockets (so accepted sockets inherit the sizes).

Changes:

• Refactors socket buffer sizing into SetSocketBufferOptions(int fd) and applies it before connect() in Socket::Connect.
• Extends butil::tcp_listen with an optional callback invoked before bind()/listen() and uses it from Server::StartInternal to set listener buffer sizes early.
• Adds unit tests verifying buffer sizes are set before connect and inherited on accept.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/brpc_socket_unittest.cpp	Adds helper routines and tests to validate buffer option timing/inheritance.
src/butil/endpoint.h	Adds BeforeListenCallback type + tcp_listen overload taking a callback.
src/butil/endpoint.cpp	Implements callback-aware tcp_listen and keeps the original wrapper overload.
src/brpc/socket.h	Declares SetSocketBufferOptions(int fd) helper.
src/brpc/socket.cpp	Moves buffer sizing into SetSocketBufferOptions and calls it before connect().
src/brpc/server.cpp	Passes SetSocketBufferOptions into tcp_listen so listener fds get sized early.

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