air: dedicated nextchanges section

[janniklasrose]

Allow the ai runtime team to stamp their own release notes

https://github.com/databricks/cli/actions/runs/35891181995?pr=6824 passes (but no-ops on the section because no fragments in the section).

Added a dummy file:

% task check-changelog
task: [check-changelog] uv run --script tools/validate_nextchanges.py

% task changelog-preview
[...]

### AI Runtime

 * Hello from air. ([#6824](https://github.com/databricks/cli/pull/6824))

[...]

[eng-dev-ecosystem-bot]

Integration test report

Commit: 1e5e35d

Run: 35891242885

	Env	✅​pass	🙈​skip	Time
✅​	aws linux	276	15	5:41
✅​	aws windows	278	13	4:52
✅​	azure linux	275	15	6:40
✅​	azure windows	277	13	5:24
✅​	gcp linux	276	15	7:31
✅​	gcp windows	278	13	5:25

Top 3 slowest tests (at least 2 minutes):

duration	env	testname
3:35	azure windows	TestAccept
3:23	gcp windows	TestAccept
3:19	aws windows	TestAccept