fix: prevent stuck disabled message buttons after stop generation (#1508)

[xielixing]

Fix: Stuck disabled message buttons after Stop Generation (#1508)

Problem

After clicking "Stop Generation" during an AI response, message action buttons (edit, rollback) become permanently unclickable. The tooltip shows "Session history is not ready yet" (editDisabledHistoryNotReady). This persists for the entire conversation and only resolves when a new conversation is started.

Root Cause

absoluteSessionTurnIndexForId in flowChatTurnOrdinal.ts returns undefined when the turn catalog (validSessionTurnCatalog) is stale or invalid -- which happens after Stop Generation interrupts async session initialization and markSessionFinished creates a new session object via spread.

When absoluteSessionTurnIndexForId returns undefined, UserMessageItem.tsx computes actionTurnIndex = -1, which triggers the !resolvedSessionId || actionTurnIndex < 0 condition that disables the edit/rollback buttons with the "history not ready" tooltip.

The sibling functions absoluteSessionTurnIndexForLocalIndex and createAbsoluteSessionTurnIndexResolver both already have a localIndex + 1 fallback for when the ordinal resolver returns undefined. Only absoluteSessionTurnIndexForId was missing this fallback.

Fix

Add a local-index fallback to absoluteSessionTurnIndexForId: when resolveTurnOrdinal returns undefined, find the turn's local index in session.dialogTurns and delegate to absoluteSessionTurnIndexForLocalIndex (which has the localIndex + 1 fallback).

This makes all three turn-index resolution functions consistent in their fallback behavior.

Validation

• tsc --noEmit on src/web-ui passes (no new type errors from this change)
• The fix is minimal -- 3 lines changed, no new dependencies, no API changes

Testing

The fix can be verified by:

1. Start a conversation and send a message
2. While the AI is responding, click "Stop Generation"
3. After generation stops, hover over the edit/rollback buttons on any message
4. The buttons should be clickable (no "Session history is not ready yet" tooltip)

Fixes #1508