fix(io): make InMemoryFileStore.list respect directory boundaries

[tomsen-ai]

HUMAN:

The in-memory file store's list() disagreed with the local (disk) one:

it invented phantom directories from sibling names, and crashed with an
IndexError when given an exact file path. I went through the
side-by-side repro before and after the fix myself.

---

AGENT:

Why

InMemoryFileStore.list uses bare string prefix matching (file.startswith(path) + removeprefix), which is not aware of the / directory boundary. It diverges from LocalFileStore.list (whose implementation is explicitly commented as S3-consistent behavior) in two observable ways:

1. Phantom directories from sibling keys. With keys sessions/abc/event.json and sessions/abcdef/other.json, list("sessions/abc") returns a phantom sessions/abc/def/ entry derived from the sibling's suffix. LocalFileStore returns only the real children.
2. Crash on exact file paths. list("sessions/abc/event.json") raises IndexError: list index out of range (suffix="" → parts=[] → parts[0]). LocalFileStore returns [path].

Both classes implement the same FileStore interface, so consumers switching between the in-memory and local backends currently see different results for identical inputs.

The in-repo consumer (conversation/event_store.py) happens to be shielded by its EVENT_NAME_RE filename filter and fixed-width event IDs, so this is not currently user-visible there — but the divergence is a trap for any other FileStore consumer.

Summary

• Short-circuit exact file hits to return [path], matching LocalFileStore.
• Normalize the search prefix to end with / so listing a/b cannot match sibling keys like a/bc.txt.
• Skip empty suffixes; keep root listing ("") behavior unchanged.
• Add a consistency test suite that runs the same fixture through both backends and asserts they agree (red on main for the two facets above, green with this change).

Issue Number

N/A (searched existing issues/PRs: #3163/#3146 are about LRU eviction in this class, not list semantics).

How to Test

uv run pytest tests/sdk/io/ -q

New tests fail on main:

FAILED test_list_exact_file_returns_path_and_does_not_raise  (IndexError)
FAILED test_list_sibling_prefix_produces_no_phantom_entries
FAILED test_list_directories_matches_local[sessions/abc]

With this change: tests/sdk/io/ 26 passed. Downstream consumer regression: tests/sdk/conversation 708 passed.

Video/Screenshots

Side-by-side on main (same fixture, same call):

InMemory list('sessions/abc'): ['sessions/abc/def/', 'sessions/abc/event.json']  <- phantom def/
Local    list('sessions/abc'): ['sessions/abc/event.json', 'sessions/abc/nested/']
InMemory list('sessions/abc/event.json'): IndexError: list index out of range
Local    list('sessions/abc/event.json'): ['sessions/abc/event.json']

After this change the two backends return identical results for all cases in the new test suite.