Make imported chat persist

[cab14bacc (Cab14bacc)]

Fixes #291986

This PR fixes the issue where imported chats are not kept. Importing a chat (via workbench.action.chat.import) opens it in a tab, but it was marked as isImported and excluded from persistence, so it never appeared in the Sessions list and was lost once the tab was closed.

Fix

In chatImportExport.ts, upgrade the imported export data into full ISerializableChatData with a fresh sessionId and creationDate before loading — the same pattern the fork action (chatForkActions.ts) already uses. The imported chat then becomes a persistent local session.

Changes

• chat/browser/actions/chatImportExport.ts – build full serializable data for imported sessions.
• chat/test/common/model/chatModel.test.ts – add a test verifying upgraded import data yields isImported === false. The existing "exported data only" test still confirms bare export data is isImported === true.

Verify

1. Create a chat in a workspace and export it to e.g. foo.json.
2. In another workspace, use Import Chat... to load foo.json.
3. The imported chat now appear alongside newer chats.
4. Close the imported chat's tab — it should remain in the Sessions list.

Discussion

I am not sure if isImported is now obsolete or not, it does still prevent exportable data from being persistent. However, it might need a rename as imported data is now persistent.

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