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

[Andy Goryachev]

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 angorya (Reviewer).

tests/system/src/test/java/test/com/sun/glass/ui/headless/HeadlessApplication2Test.java line 77:

> 75:             fxThread.join(10);
> 76:         }  catch (InterruptedException e) {
> 77:             fail.set(true);

`Assertions.fail()` might work too

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PR Review: https://git.openjdk.org/jfx/pull/1934#pullrequestreview-3336283309
PR Review Comment: https://git.openjdk.org/jfx/pull/1934#discussion_r2429616805