net: preserve socket errors across syscall boundaries

[xuchang-vivo]

Summary

• Map ConnectionError and SocketError variants to specific negative POSIX errno values.
• Preserve those errors through socket syscalls instead of collapsing every failure to -1.
• Preserve socket read/write errors through sockfs instead of returning the generic code::ERROR.
• Add unit coverage for common mappings and nested error forwarding.

Motivation

The BlueOS network stack already distinguishes conditions such as timeout, would-block, invalid file descriptor, queue exhaustion, and address conflicts. The socket syscall and VFS boundaries previously collapsed these failures to a generic -1, losing the original reason before it reached libc.

For example, a timeout should leave the kernel as -ETIMEDOUT, allowing the libc wrapper to return -1 with errno = ETIMEDOUT. Returning a bare -1 from the kernel instead makes the libc layer interpret the error number as 1 or observe stale errno.

This complements vivoblueos/librs#38, which converts negative kernel errno results to the POSIX libc -1 plus errno convention.

This PR only preserves reported errors. It does not implement socket timeout enforcement or change network operation behavior.

POSIX references

• errno
• send()
• recv()

Testing

• Added unit tests for connection and socket errno mappings.
• rustfmt --check kernel/src/net/connection_err.rs kernel/src/net/socket/socket_err.rs kernel/src/net/syscalls.rs kernel/src/vfs/sockfs.rs
• git diff --check
• No board build was run.

[xuchang-vivo]

build_prs

[github-actions]

Job is started, see https://github.com/vivoblueos/kernel/actions/runs/32947049817.

[github-actions]

✅ All jobs completed successfully, see https://github.com/vivoblueos/kernel/actions/runs/32947049817.