VSCode extension: session shows "Chat processing error" then resets to blank welcome screen after Cmd+Q relaunch

**Steps to reproduce:**
1. Work in a session using the Claude Code VSCode extension
2. Quit the app with Cmd+Q
3. Reopen

**Observed:**
The tab is labeled "Chat processing error" with an empty pane. Reopening/refocusing lands on the default new-chat welcome screen instead of resuming the prior conversation.

**Confirmed NOT data loss:**
The session's `.jsonl` transcript on disk kept saving and growing normally during a prior occurrence of this bug — the issue appears to be in the extension's session-resume/error-state UI, not in persistence.

**Frequency:**
Recurred 3 times in one day on the same machine.

**Environment:**
- macOS Darwin 24.6.0
- Claude Code VSCode extension

Reproduce the Cmd+Q relaunch flow in the Claude Code VSCode extension and start by examining the session-resume and error-state UI. Compare the UI behavior with the session's persisted .jsonl transcript; done means an existing conversation resumes instead of showing a processing error or blank welcome screen.

Written by the indexing model from the issue text.