fix(webapi): correct binding nullability and exception contracts [Codex] (#349)

## Summary

- Corrects nullability and optionality across affected Web API bindings.
- Adds `@throws(JsExn)` where synchronous browser APIs can raise
exceptions.
- Models `HTMLElement.hidden` with the semantic states `Visible`,
`Hidden`, and `HiddenUntilFound`, without a raw coercion API.
- Updates compile-time fixtures for the corrected contracts.

## `HTMLElement.hidden`

The getter and setter use the same semantic type:

```rescript
element->HTMLElement.setHidden(HTMLElement.Hidden)
element->HTMLElement.setHidden(HTMLElement.HiddenUntilFound)
element->HTMLElement.setHidden(HTMLElement.Visible)

switch element->HTMLElement.hidden {
| HTMLElement.Visible => "visible"
| HTMLElement.Hidden => "hidden"
| HTMLElement.HiddenUntilFound => "hidden until found"
}
```

Because `setHidden` is an `@set` external, these calls emit direct
property assignments such as `element.hidden = true`.

An earlier version also exposed `hiddenInput` and `setHiddenRaw` to
mirror every Web IDL setter input. Those were removed because numbers,
arbitrary strings, `null`, and `undefined` only invoke coercion and do
not represent additional states. The binding intentionally restricts
writes to the three useful outcomes. See
[MDN](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/hidden)
and the [HTML
Standard](https://html.spec.whatwg.org/multipage/interaction.html#dom-hidden).

Author: jderochervlk

src/animation/Animation.res

@@ -87,7 +87,7 @@ external cancel: t => unit = "cancel"
 /**
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Animation/finish)
 */
-@send
+@throws(JsExn) @send
 external finish: t => unit = "finish"
 
 /**
@@ -99,7 +99,7 @@ external play: t => unit = "play"
 /**
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/Animation/pause)
 */
-@send
+@throws(JsExn) @send
 external pause: t => unit = "pause"
 
 /**
@@ -138,12 +138,12 @@ type iterationCompositeOperation =
 type keyframeEffectOptions = {
   ...AnimationEffect.effectTiming,
   mutable composite?: compositeOperation,
-  mutable pseudoElement?: Null.t<string>,
+  mutable pseudoElement?: Nullable.t<string>,
   mutable iterationComposite?: iterationCompositeOperation,
 }
 
 type keyframeAnimationOptions = {
   ...keyframeEffectOptions,
   mutable id?: string,
-  mutable timeline?: Null.t<animationTimeline>,
+  mutable timeline?: Nullable.t<animationTimeline>,
 }

src/animation/AnimationEffect.res

@@ -32,12 +32,12 @@ type getAnimationsOptions = {mutable subtree?: bool}
 
 type computedEffectTiming = {
   ...effectTiming,
-  mutable progress?: Null.t<float>,
-  mutable currentIteration?: Null.t<float>,
+  mutable progress?: Nullable.t<float>,
+  mutable currentIteration?: Nullable.t<float>,
   mutable startTime?: float,
   mutable endTime?: float,
   mutable activeDuration?: float,
-  mutable localTime?: Null.t<float>,
+  mutable localTime?: Nullable.t<float>,
 }
 
 type optionalEffectTiming = {
@@ -67,5 +67,5 @@ external getComputedTiming: t => computedEffectTiming = "getComputedTiming"
 /**
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/AnimationEffect/updateTiming)
 */
-@send
+@throws(JsExn) @send
 external updateTiming: (t, ~timing: optionalEffectTiming=?) => unit = "updateTiming"

src/canvas/CanvasGradient.res

@@ -4,7 +4,7 @@ Adds a color stop with the given color to the gradient at the given offset. 0.0
 Throws an "IndexSizeError" DOMException if the offset is out of range. Throws a "SyntaxError" DOMException if the color cannot be parsed.
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/CanvasGradient/addColorStop)
 */
-@send
+@throws(JsExn) @send
 external addColorStop: (CanvasTypes.canvasGradient, ~offset: float, ~color: string) => unit =
   "addColorStop"