fix: preserve signature of observable function children (#631, #521)

[trinadhkoya]

Summary

Fixes #631 and #521 — both stem from the same reduction in the ObservableFunctionChildren type.

Function children stored in an observable that return a non-void value had their signature reduced to () => t:

• ObservableFunctionChildren strips argument signatures from non-void functions #631 — the argument list was dropped, so calling the function was a type error:

  type AppState = { map: { centerMap: (coords: unknown) => boolean; zoom: number } };
  const appState$ = observable<AppState>({ /* ... */ });
  appState$.map.centerMap(data); // ❌ Expected 0 arguments, but got 1

• async function without args in observable child is unwrapped (Promise<T> becomes T). Possibly type inference error? #521 — an async function child with no arguments had its Promise<T> return unwrapped to T:

  const example$ = observable<{ asyncFn(): Promise<number> }>({ async asyncFn() { return 1; } });
  example$.asyncFn(); // was inferred as number, should be Promise<number>

Fix

In src/observableTypes.ts, ObservableFunctionChildren now intersects with the original function type T[K] instead of () => t, preserving the full call signature and return type:

- : Observable<t> & (() => t)
+ : Observable<t> & T[K]

This is the fix proposed by the original reporter in #631.

Tests

Added type tests in tests/types.test.ts:

• issue #631 — asserts the argument signature is preserved (fails on the old type with Expected 0 arguments, but got 1).
• issue #521 — asserts an async no-arg child keeps its Promise<number> return type.

Both tests fail against the previous type and pass with the fix.

Verification

• npm run lint:check ✅
• npm run format:check ✅
• npm run typecheck ✅ (fails without the fix, confirming the new tests guard it)
• npm run build ✅