fix(sessions): raise SessionNotFoundError when appending to an unknown Redis session

[Hugo-DDT]

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Link to Issue or Description of Change

2. No issue exists; description of the change:

Problem:
RedisSessionService.append_event writes the session key unconditionally, so appending an event to a session that was never created (or was deleted/expired) silently creates it instead of raising SessionNotFoundError like InMemorySessionService, SqliteSessionService and DatabaseSessionService do. App- and user-scoped state deltas from such an event were also written before the session write. The shared session-contract suite already records this as a known divergence: tests/unittests/sessions/_conformance.py marks test_append_event_to_unknown_session_raises_session_not_found as xfail(strict=True) for the redis backend, noting that whoever fixes the backend must delete the entry.

Repro on main:

pytest tests/unittests/sessions/test_session_service.py -k unknown_session_raises --runxfail
FAILED ...::test_append_event_to_unknown_session_raises_session_not_found[redis] - Failed: DID NOT RAISE SessionNotFoundError

Solution:
Check that the session key exists before any write (skipped for partial events, which return early as in the other services) and raise SessionNotFoundError otherwise. Checking first means a rejected append leaves no app/user state behind. The existence check uses EXISTS rather than GET, so the existing concurrent-change retry test still exercises the transaction path. The divergence entry is removed, so the contract test now runs strictly for Redis.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.

• All unit tests pass locally.

• New test_append_event_to_unknown_session_writes_nothing (Redis): raises, and nothing is stored.

• The shared contract test now passes for redis (xfail entry removed).

• pytest tests/unittests/sessions tests/unittests/integrations/redis: 615 passed (Python 3.11 and 3.13).

• FakeRedisAsync gains an exists() mirroring redis-py's.

Manual End-to-End (E2E) Tests:

N/A: storage-layer change covered by the shared session contract tests; no UI or runner behavior change for existing sessions.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

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