fix: new terminals will be created in the panel of the clicked button

[Fangoling]

What it does

Previously, clicking the "Add Terminal" button in the bottom panel would always create a terminal widget in the bottom panel of the "current terminal".

Now, clicking the "Add Terminal" button in a bottom panel will create a terminal widget in the same bottom panel, even when the "current terminal" is in another bottom panel.

Fixes #16527

How to test

1. Open a terminal in the bottom panel.
2. Click the split terminal button on the bottom panel to open a second bottom panel.
3. Click on the terminal in the left panel to focus it.
4. Click on the add terminal button on the right panel

Follow-ups

In the function splitTerminal (line 1003) of terminal-frontend-contribution.ts, the result of the withWidget call is checked using the || operator instead of an explicit === false comparison.
Since withWidget returns a value of type T | false, using === false would make the condition clearer and more type-safe.

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Review checklist

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

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