fix(types): allow partial statics in Schema.static()

[maynkxx]

Fixes #15780

Problem

When using Schema.static({...}) with TypeScript generics, TypeScript required
that all properties defined in TStaticMethods be present in the object passed
to schema.static(). This prevented users from adding statics one at a time.

Solution

Make the mapped type keys optional:
{ [F in keyof TStaticMethods]?: TStaticMethods[F] }

This aligns Schema.static() with the behavior of Schema.method() and allows
incremental addition of static methods.

Notes

• This change affects only TypeScript definitions.
• No runtime behavior is touched.
• Low risk, minimal change.

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[vkarpov15]

I don't like the direction of this PR. Our current thinking is that we want to err on the side of being more strict with our types, because it is fairly easy to work around cases like this using as.

In the case of statics, if you want to define them one at a time, you can just use the static(name, fn) signature.

@hasezoey @AbdelrahmanHafez WDYT?

[hasezoey]

From a pure code/type style standpoint, this looks good to me, maybe consider adding a types test though.

Otherwise, i dont really have a preference in this case and either would be fine for me.