fix(core/layers): handle non-trailing-slash path in native recursive list

[tonghuaroot]

Which issue does this PR close?

Related to #4256. This PR does not close that issue; it fixes a latent
correctness bug in the recursive-list path that was found while investigating
#4256, and is a prerequisite for any future native WebDAV recursive-list work.

Rationale for this change

SimulateAccessor::simulate_list has three arms. Two of them (the
simulated-recursive arm (true, false, true) and the non-recursive arm
(false, false, _)) already special-case a list path without a trailing slash:
they list the parent directory and wrap the result in a PrefixLister, so that
listing dir/file returns every entry sharing that prefix (for example
dir/file and dir/file2).

The native-recursive arm (_, true, _) did not. It forwarded the path straight
to the backend. On a backend that natively supports recursive list and follows
directory/subtree semantics (a WebDAV server honoring Depth: infinity),
listing dir/file walks the subtree of dir/file and returns only that one
entry, silently dropping prefix-siblings such as dir/file2. The result of a
recursive list then depends on whether the backend's native lister happens to
be prefix-based or subtree-based, which is inconsistent.

The in-memory service did not expose this because its native lister is
prefix-based (scan(prefix) returns every key that starts with the prefix), so
the bug stayed latent until a real WebDAV backend with native Depth: infinity
exercised it.

What changes are included in this PR?

• In the (_, true, _) arm of simulate_list: when the path has a trailing
  slash, forward to the backend as before; when it does not, list the parent
  directory and wrap the result in a PrefixLister, matching the other two
  arms.
• A focused regression test (mod tests in simulate.rs) using a mock backend
  that models WebDAV Depth: infinity semantics. It asserts that recursively
  listing dir/file (no trailing slash) returns both dir/file and
  dir/file2. The test fails without this change and passes with it. A second
  case asserts that listing a trailing-slash directory path is unchanged.

Are there any user-facing changes?

Yes, a bug fix: for services that declare native recursive list, calling a
recursive list with a path that has no trailing slash now correctly returns all
entries sharing that path prefix, consistent with non-native-recursive
services. No API changes. No breaking changes.

AI Usage Statement

This change was co-authored with AI assistance. The contributor drove the work:
the bug was found while reproducing #4256 against real WebDAV servers
(nginx, ownCloud, Nextcloud) in Docker, where a recursive list of a
non-trailing-slash path against a native Depth: infinity backend dropped a
sibling entry. The contributor diagnosed the root cause in simulate.rs,
verified the fix and the regression test against the memory service and a real
Nextcloud backend, and reviewed the final implementation. An AI assistant
(Claude) helped implement the fix and draft the regression test and this
description.