Fix flush() to handle VirtualPipe on stdio fds

In nested subshells, fd 1 (stdout) may be replaced with a VirtualPipe
from an outer subshell's dup2 operation. When fd_renumber tries to flush
fd 1 before replacing it, stdout_mut() returns FsError::NotAFile because
VirtualPipe is not Kind::File.

This fix makes flush() handle the NotAFile case gracefully for stdio fds,
since pipes don't require explicit flushing. This fixes nested backticks
in bash command substitution:

  echo `echo \`echo hi\``  # Previously failed with "Invalid argument"

The $() command substitution still fails because bash requires /dev/fd
which is not yet implemented in WASIX.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: NathanFlurry

lib/wasix/src/fs/mod.rs

@@ -1565,14 +1565,29 @@ impl WasiFs {
         match fd {
             __WASI_STDIN_FILENO => (),
             __WASI_STDOUT_FILENO => {
-                let mut file =
-                    WasiInodes::stdout_mut(&self.fd_map).map_err(fs_error_into_wasi_err)?;
-                file.flush().await.map_err(map_io_err)?
+                // stdout might have been replaced with a pipe in subshells
+                // Check the Kind first before trying to flush as a File
+                match WasiInodes::stdout_mut(&self.fd_map) {
+                    Ok(mut file) => file.flush().await.map_err(map_io_err)?,
+                    Err(FsError::NotAFile) => {
+                        // fd 1 was replaced with a non-File (e.g., VirtualPipe)
+                        // Pipes don't need explicit flushing, so just succeed
+                        ()
+                    }
+                    Err(e) => return Err(fs_error_into_wasi_err(e)),
+                }
             }
             __WASI_STDERR_FILENO => {
-                let mut file =
-                    WasiInodes::stderr_mut(&self.fd_map).map_err(fs_error_into_wasi_err)?;
-                file.flush().await.map_err(map_io_err)?
+                // stderr might have been replaced with a pipe in subshells
+                match WasiInodes::stderr_mut(&self.fd_map) {
+                    Ok(mut file) => file.flush().await.map_err(map_io_err)?,
+                    Err(FsError::NotAFile) => {
+                        // fd 2 was replaced with a non-File (e.g., VirtualPipe)
+                        // Pipes don't need explicit flushing, so just succeed
+                        ()
+                    }
+                    Err(e) => return Err(fs_error_into_wasi_err(e)),
+                }
             }
             _ => {
                 let fd = self.get_fd(fd)?;