Remove minItems constraint from RunAfter.items in Chaos preview versions (#45380)

The spec declared minItems: 1 on RunAfter.items, but the service backend
never enforced this constraint. The divergence was latent until a recent
model simplification regenerated validation from the spec, which began
enforcing minItems and broke the Azure portal for scenarios that send
runAfter with an empty items array.

This realigns the spec with actual, long-standing service behavior.

Affects 2026-05-01-preview and 2026-08-01-preview only. The stable
2025-01-01 version is unaffected because RunAfter does not exist there.

Co-authored-by: Julia Gao <juliagao@microsoft.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>