net: reject invalid fds before wrapping Socket

[sankalpsthakur]

Summary

new net.Socket({ fd }) can abort the process when callers pass arbitrary or already-in-use descriptors. On Linux this showed up as:

node: ../deps/uv/src/unix/linux.c:1427: uv__io_poll: Assertion `errno == EEXIST' failed.

after epoll_ctl(ADD) returned EBADF because a wrapped fd was closed before the watcher was registered.

This PR:

• Validates the fd with fstat in createHandle() so closed/invalid descriptors throw a JS ErrnoException (EBADF) instead of reaching libuv.
• Extends uv__fd_exists() so loop-private fds (backend, signal pipe, async pipe) are treated as already in use (EEXIST on open).
• Softens the Linux epoll_ctl path to treat EBADF / ENOENT / EPERM as recoverable instead of aborting.

Fixes #63308

Validation

• node --check lib/net.js
• node --check test/parallel/test-net-socket-invalid-fd.js
• Reproduced the abort on Linux (node:24-bookworm-slim) with the issue's fd-iteration snippet; root cause confirmed via strace (epoll_ctl ... EBADF then the errno == EEXIST assert).
• Full ./node rebuild was too heavy in this environment (no local Release binary for this worktree; incremental rebuild pulled V8). New coverage is in test/parallel/test-net-socket-invalid-fd.js for CI.

AI/LLM disclosure

• AI coding tools (including Grok and/or Codex agent-assisted editing) were used to help draft or modify code and this PR description.
• I reviewed the complete change, understand the reasoning, and ran the reported local tests before submitting.
• This submission is original work of authorship under the project CLA / contributor terms; AI output was not pasted unreviewed.

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[nodejs-github-bot]

Review requested:

• @nodejs/net
• @nodejs/security-wg