FIX Correlate multi-turn attack error results

[romanlutz]

Description

PR #1720 added generic persistence for uncaught attack exceptions. That handler looked for context.conversation_id, which is correct for SingleTurnAttackContext, but multi-turn attacks store the active ID at context.session.conversation_id. For every terminal multi-turn exception, the lookup therefore failed and the handler generated a new UUID.

If turns had already been stored under conversation A, the resulting AttackOutcome.ERROR row was persisted under unrelated conversation B:

• The conversation messages remained intact under A.
• The AttackResult was valid but incorrectly referenced B.
• The database accepted the mismatch because conversation_id is a string rather than a foreign key, and a legitimate attack may fail before writing any messages.
• Backend history and frontend views then queried statistics for B, commonly reporting zero messages and no preview while the partial conversation under A was disconnected from the result.

This is logical record corruption rather than malformed rows or lost message data. It affects only uncaught terminal exceptions in multi-turn attacks since #1720; successful attacks, ordinary non-exception outcomes, and single-turn errors are unaffected. Existing affected rows cannot be repaired safely in a general migration because a generated ID from this bug is indistinguishable from a legitimate pre-first-message failure without attribution, logs, or other execution evidence identifying the original conversation.

This change resolves the conversation ID from the direct context field first, then from the multi-turn conversation session, retaining UUID generation only for contexts that expose neither layout.

Tests and Documentation

• Added a regression test using a real MultiTurnAttackContext and ConversationSession to verify timeout error results retain the active conversation ID.
• Ran the attack strategy and Crescendo unit suites: 127 passed.
• Ran the dataset, Many-shot, and scenario-technique invariant suites: 4,688 passed.
• Ruff, formatting, ty, repository commit hooks, and git diff --check passed.
• Documentation: N/A; this corrects internal error-result correlation without changing the public API.