Redis SessionStore can silently lose or duplicate appends

The Redis `SessionStore` reference adapter ignores per-command errors returned
by `MULTI/EXEC`.

Two failure modes follow:

1. If the transcript key has the wrong type, `append()` resolves even though
`RPUSH` failed.
2. If the index key has the wrong type, `RPUSH` succeeds before the index
command fails. Retrying the same append duplicates transcript entries.

Reproduced on
https://github.com/anthropics/claude-agent-sdk-typescript/commit/e3d162bdf035169d0ea44bb6314d6e8fc5c730fe
against Redis 7:

```text
append rejects Redis transaction command errors:
Expected promise that rejects
Received promise that resolved

three SDK retries of an index failure:
Expected []
Received three duplicate transcript entries
```

Checking the `EXEC` result surfaces errors but does not make the write safe,
because Redis does not roll back commands that succeeded earlier in the
transaction. A failure-atomic fix needs to validate both key types before
writing them in one server-side operation.

Start at the Redis SessionStore append implementation and inspect how MULTI/EXEC handles per-command results from RPUSH and the index update. Reproduce the wrong-key-type cases on Redis 7, then verify that an append rejects failures without leaving partial writes or duplicate transcript entries during retries.

由索引模型根據 Issue 內容生成。