Persist active tab state in diagnostic view

[Copilot]

Diagnostic view resets to the first tab on every navigation, forcing users to re-select their tab and causing unnecessary reloads of the Session Files content.

Changes

• State management: Use VS Code webview state API to persist activeTab across view lifecycle
• Tab activation: Extract tab switching logic into activateTab(tabId) helper that returns success/failure
• Restoration: On render, restore saved tab with fallback to default if tab doesn't exist

Implementation

// Save on tab click
vscode.setState({ activeTab: tabId });

// Restore on view load
const savedState = vscode.getState();
if (savedState?.activeTab && !activateTab(savedState.activeTab)) {
    activateTab('report');  // fallback
}

State persists even with retainContextWhenHidden: false since it uses the webview state API rather than in-memory storage.

Original prompt

Every time we go back to the diagnostic view it starts on the first tab instead of staying open on the last opened tab.

That means also that the files sub tab is reloaded as well, instead of just shown!

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