SMB Server Phase 2: VFS backend build fix + integration test

- Add VfsFile: Send supertrait for Mutex compatibility
- Fix SmbServerCommand: struct → Subcommand enum with Start variant
- Fix tracing_subscriber::init() → try_init() to avoid panic when
  logger already initialized
- Fix CLI subcommand name: smb-server → smb-start (flatten naming)
- Add #[command(name = "smb-start")] for CLI disambiguation
- Fix unused variable warnings (smb_fs.rs, smb_server_backend.rs)
- Remove unused VfsFile imports (webdav.rs, scp_handler.rs)
- Integration test: Docker smbclient verified (list, upload, read)

vendor/smb-server/src/proto/messages/error_response.rs

@@ -0,0 +1,84 @@
+//! SMB2 ERROR Response (MS-SMB2 §2.2.2).
+//!
+//! Sent in place of any normal response when the server returns a non-zero
+//! NTSTATUS. The SMB2 header carries the NTSTATUS in `channel_sequence_status`;
+//! this body provides extended error context if any.
+
+use binrw::{BinRead, BinWrite, binrw};
+use std::io::Cursor;
+
+use crate::proto::error::ProtoResult;
+
+/// MS-SMB2 §2.2.2 ERROR Response.
+///
+/// `structure_size` is always 9; `byte_count` is the length of `error_data`
+/// when there is no structured error context (the common case). When
+/// `error_context_count > 0`, `error_data` holds a sequence of
+/// [`ErrorContext`] entries (SMB 3.1.1+).
+#[binrw]
+#[brw(little)]
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ErrorResponse {
+    pub structure_size: u16,
+    pub error_context_count: u8,
+    pub reserved: u8,
+    pub byte_count: u32,
+    #[br(count = if byte_count == 0 { 1 } else { byte_count as usize })]
+    pub error_data: Vec<u8>,
+}
+
+impl ErrorResponse {
+    /// Build a minimal ERROR response body for the given NTSTATUS.
+    ///
+    /// Per MS-SMB2 §2.2.2 a zero-`byte_count` ERROR response still emits a
+    /// single byte of `error_data` (the field is mandatory, length 1 when
+    /// there is no payload).
+    pub fn status(_ntstatus: u32) -> Self {
+        Self {
+            structure_size: 9,
+            error_context_count: 0,
+            reserved: 0,
+            byte_count: 0,
+            error_data: vec![0],
+        }
+    }
+
+    pub fn parse(buf: &[u8]) -> ProtoResult<Self> {
+        let mut c = Cursor::new(buf);
+        Ok(Self::read(&mut c)?)
+    }
+
+    pub fn write_to(&self, out: &mut Vec<u8>) -> ProtoResult<()> {
+        let mut c = Cursor::new(Vec::new());
+        BinWrite::write(self, &mut c)?;
+        out.extend_from_slice(&c.into_inner());
+        Ok(())
+    }
+}
+
+/// MS-SMB2 §2.2.2.1 ERROR Context Response (3.1.1+).
+#[binrw]
+#[brw(little)]
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct ErrorContext {
+    pub error_data_length: u32,
+    pub error_id: u32,
+    #[br(count = error_data_length as usize)]
+    pub error_context_data: Vec<u8>,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn round_trips_status_helper() {
+        let r = ErrorResponse::status(0xC000_0022 /* STATUS_ACCESS_DENIED */);
+        let mut buf = Vec::new();
+        r.write_to(&mut buf).unwrap();
+        let decoded = ErrorResponse::parse(&buf).unwrap();
+        assert_eq!(decoded, r);
+        // structure_size, contexts, reserved, bytecount, 1 byte payload = 9 bytes
+        assert_eq!(buf.len(), 9);
+    }
+}