fix(terminal-manager): update active page handling on terminal page deletion

[Kongkon06]

What it does

Fixes #16758

This PR improves Terminal Manager behavior when terminal.grouping.mode is set to tree by:

• Preventing the creation of a new empty page when the last terminal page is deleted.
• Ensuring a remaining page is automatically selected after a page deletion.
• Fixing the Reset Layout command to properly clear and reset the terminal layout.

To support this, the PR introduces a small internal flag isResetingLayout and helper method resetView in
terminal-manager-widget to distinguish between explicit layout resets and user-initiated page deletions, ensuring correct
lifecycle handling without changing public APIs.

How to test

Ensure terminal.grouping.mode is set to tree in your preferences.

Test 1: Delete the last page

1. Open the Terminal Manager with a single terminal page.
2. Delete the page.

Expected result:

• The Terminal Manager widget closes.
• No new empty page is created.

Test 2: Auto-selection after page deletion

1. Open the Terminal Manager with at least two terminal pages.
2. Delete one page.

Expected result:

• A remaining page is automatically selected.
• A terminal remains visible.

Test 3: Reset Layout

1. Open the Terminal Manager and create multiple terminal pages.
2. Run Terminal Manager: Reset Layout and confirm.

Expected result:

• All terminal pages are cleared.
• The terminal layout is reset correctly (Also creates a fresh page).

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and followed the review guidelines the review guidelines
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