fix(session): compare message positions instead of IDs in SessionPrompt.run

[alfredocristofano]

Issue for this PR

Closes #23490

Type of change

• Bug fix

What does this PR do?

Replaces lexicographic message ID comparisons with array index comparisons in SessionPrompt.run. Custom message IDs broke loop exit logic because lastUser.id < lastAssistant.id does not reflect actual transcript order.

Changes:

• Tracks lastUserIndex, lastAssistantIndex, and lastFinishedIndex while scanning messages from the end.
• Replaces lastUser.id < lastAssistant.id with lastUserIndex < lastAssistantIndex.
• Replaces the for...of loop that skipped messages via m.info.id <= lastFinished.id with a for loop starting from lastFinishedIndex + 1.

How did you verify your code works?

The logic correctly handles custom message IDs by relying on array positions, which always reflect the actual chronological order.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicate Found

PR #24379: fix(session): use transcript position instead of lexical ID compare in prompt loop

• fix(session): use transcript position instead of lexical ID compare in prompt loop #24379

Why it's related: This PR appears to address the same issue - replacing lexical ID comparisons with transcript position/array index comparisons in the SessionPrompt loop logic. Both PRs are fixing the same problem where custom message IDs broke loop exit logic by not reflecting actual transcript order.