RFR: 8369306: Implement invokeAndWait and finishTerminating on headless platform [v4]

[Kevin Rushforth]

On Tue, 14 Oct 2025 08:24:36 GMT, Jose Pereda <jpereda at openjdk.org> wrote:

>> This PR implements `invokeAndWait` and `finishTerminating` for the headless platform and includes two system tests, one for each, that fail before this PR, and pass after it.
>
> Jose Pereda has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Process feedback

Marked as reviewed by kcr (Lead).

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PR Review: https://git.openjdk.org/jfx/pull/1934#pullrequestreview-3336366489