fix(chat): handle non-array children in fallback text and Actions types

[Ehesp]

Hello,

This PR fixes two related issues that showed up when using cards built with JSX or after serialization, and when nesting Actions([...]) inside Card({ children: [...] }).

1. Runtime crash

For section and fields nodes, the fallback-text path assumed children was always an array and called child.children.map(...). After JSX or serialization, children can be a single value or undefined, which led to element.children.map is not a function.

2. Type error when nesting Actions

Using the documented array form Actions([Button(...), Button(...)]) inside Card({ children: [...] }) caused a type error. The inner array was inferred as CardChild[], so it didn’t match the strict action-children overload and also didn’t match ContainerProps, giving: "Type 'ChatElement[]' has no properties in common with type 'ContainerProps'":

Essentially:

// No type errors, but runtime error
Actions({
  children: [
    Button({ id: "approve", label: "Approve", style: "primary" }),
    Button({ id: "reject", label: "Reject", style: "danger" }),
  ],
}),

// Type errors, but no runtime crash (and works as documented)
Actions([
    Button({ id: "approve", label: "Approve", style: "primary" }),
    Button({ id: "reject", label: "Reject", style: "danger" }),
]),

This PR does:

1. Introduces getChildrenArray(node), which always returns an array: it keeps arrays as-is, wraps a single child in [child], and returns [] when children is missing or null. It uses it everywhere we iterate over children in the fallback-text path (in both cards.ts and markdown.ts’s BaseFormatConverter), so we never call .map on a non-array.

2. Updates ActionsComponent so the array form is a single overload that accepts
   readonly CardChild[] | readonly ActionsChild[] | ActionsChild[]. That way Actions([...]) type-checks both on its own and when nested in Card({ children: [...] }), without adding overloads that would trigger the “unified type signatures” lint.

Test plan

Added tests to PR.

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