Improve terminal creation handling & grouping in TerminalManager

[colin-grant-work]

What it does

Closes #17411
Closes #17157

Adjusts the way the terminal manager takes ownership of terminals by implementing a contribution system to handle placement of terminals in the layout. This eliminates various mechanisms used by the terminal manager to try to distinguish between terminals it opened itself and those opened by other callers.

How to test

Follow testing from #17403

Follow-ups

Breaking changes

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

Attribution

Review checklist

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

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[colin-grant-work]

Marking as a draft while I look at #17157

[colin-grant-work]

To address #17157, I ended up making some changes upstream of the terminal manager in core's tree utilities. I think those changes are correct and the old behavior was just buggy / leaky (if mostly benign), but if we prefer not to change the core tree utilities, the problems can be worked around more locally.