RFR: 8368856: Add a method to safely add duration to instant [v4]

[Pavel Rappo]

> We recently [discussed] the possibility of introducing saturating arithmetic for deadline computation. Consider this PR as a starting point. Once we agree on the implementation, I'll file a CSR.
> 
> I created a method in `Instant` to add `Duration`. One could argue that the proper way would be to go all the way and create a method in `Temporal` to add `TemporalAmount`. Or maybe even expand the functionality, and create an additional method in `Temporal` to subtract `TemporalAmount`.
> 
> My current thinking is that if we were to do that, there would be a lot of expensive, unused code. Saturating logic seems to be only useful for `Instant` and `Duration`.
> 
> Even if we decide to extend `Temporal` to add/subtract `TemporalAmount`, it could always be done later. From the perspective of `Instant`, `plus(TemporalAmount)` will be just an overload of `plus(Duration)`.
> 
> [discussed]: https://mail.openjdk.org/pipermail/core-libs-dev/2025-September/151098.html

Pavel Rappo has updated the pull request incrementally with two additional commits since the last revision:

 - Strengthen the test
 - Make new test conform to prevailing test style

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/27549/files
  - new: https://git.openjdk.org/jdk/pull/27549/files/6fec3225..1fe394b4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27549&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27549&range=02-03

  Stats: 54 lines in 1 file changed: 17 ins; 0 del; 37 mod
  Patch: https://git.openjdk.org/jdk/pull/27549.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27549/head:pull/27549

PR: https://git.openjdk.org/jdk/pull/27549