[REFACTOR] FSAL typed error enum to replace substring-based NFS error mapping

[amarok-60T]

Background

NFS handlers in src/nfs/*.rs currently map FSAL errors to nfsstat3 codes by
substring-matching error.to_string(). The pattern looks like:

let error_status = if e.to_string().contains("not found")
    || e.to_string().contains("Invalid handle")
{
    nfsstat3::NFS3ERR_STALE
} else {
    nfsstat3::NFS3ERR_IO
};

This was introduced opportunistically during the multi-export work (#26)
and now appears in roughly 14 sites across the NFS handlers, with subtly
different substrings ("not found", "No such", "no such file",
"Invalid handle", "cross-device" etc.). Each new FSAL error type
risks a silent fall-through to NFS3ERR_IO.

Scope

Replace the substring matchers with a typed FSAL error so handlers
classify errors at the type level:

pub enum FsalError {
    NotFound,
    InvalidHandle,
    PermissionDenied,
    ReadOnly,
    CrossDevice,
    NotSupported,
    Other(String),
}

• Convert LocalFilesystem / MultiExportFilesystem methods to return
  Result<T, FsalError> instead of anyhow::Result<T>.
• Update each NFS handler to match the error variant directly:

  match filesystem.getattr(&handle).await {
      Ok(attr) => ...,
      Err(FsalError::NotFound | FsalError::InvalidHandle) =>
          return create_pathconf_error(xid, nfsstat3::NFS3ERR_STALE),
      Err(FsalError::PermissionDenied) =>
          return create_pathconf_error(xid, nfsstat3::NFS3ERR_ACCES),
      Err(_) => return create_pathconf_error(xid, nfsstat3::NFS3ERR_IO),
  }

• Remove the ~14 substring-match sites and the temporary helper at
  src/nfs/error.rs::classify_handle_error introduced by [FEATURE] Support multiple NFS exports #26.

Acceptance criteria

• FsalError enum exists with variants covering every wire-mappable
  error category currently surfaced (NFS3ERR_STALE, NFS3ERR_IO,
  NFS3ERR_ACCES, NFS3ERR_ROFS, NFS3ERR_XDEV, NFS3ERR_NOTSUPP,
  NFS3ERR_PERM, etc.).
• All Filesystem and ExportRegistry trait methods return
  Result<T, FsalError>.
• LocalFilesystem and MultiExportFilesystem impls map IO/syscall
  errors into the correct variants (e.g. io::ErrorKind::NotFound
  → FsalError::NotFound).
• Every e.to_string().contains(...) call site in src/nfs/ is
  replaced with a type-level match.
• src/nfs/error.rs::classify_handle_error (the multi-export
  transitional helper) is removed.
• make test and make nfstest pass.
• No regression in wire-level NFS status codes returned to clients —
  add a Python integration test that triggers each error category if
  one doesn't already exist.

References

• The multi-export PR introduced the helper that this issue is meant to
  retire: #27
• Sites currently using substring matchers (non-exhaustive grep at config: introduce per-export configuration #27 head):
  src/nfs/access.rs, src/nfs/commit.rs, src/nfs/create.rs,
  src/nfs/fsstat.rs, src/nfs/fsinfo.rs, src/nfs/lookup.rs,
  src/nfs/pathconf.rs, src/nfs/read.rs, src/nfs/setattr.rs,
  src/nfs/write.rs, src/nfs/rename.rs, src/nfs/link.rs, etc.
• Phase 3 review identified this as a known fragility:

  "NFS handler 用 substring match 抓 error 換 typed enum"

• Phase 4 / Phase 5 deliberately kept substring matching to avoid
  scope creep on PR config: introduce per-export configuration #27.

Out of scope (do separately)

• Restructuring the broader FSAL trait hierarchy (this issue assumes the
  trait shape from [FEATURE] Support multiple NFS exports #26 stays).
• Adding new NFS error mappings beyond what is already returned today
  (e.g. richer ACL errors).