[Chore]: `@ag-ui/client`|`@ag-ui/core`, bundle size optimization: Use treeshakeable Zod

[louis-bompart]

Originally discussed on Discord

The @ag-ui/core & @ag-ui/client use Zod for schema validation.
By default, and in v3, it is not tree-shakeable, meaning terser & bundler cannot easily remove unused code coming from zod.

To solve this, Zod introduced a new "flavour" of Zod in its v4, Zod Mini (See release announcement

In louis-bompart/ag-ui#4, I did the experiment of updating Zod to 3 & shifting all use from Zod "classic" to Zod Mini.

This resulted in a reduction of ~84kb uncompressed, which I estimate represents a ~22% improvement for the bundle size of a very simple HTTPAgent.

To note, numbers are less impressive in a more realistic scenario where the output is gzipped; we shave only 8kb (which still represents a ~13% improvement for the bundle size)

louis-bompart/ag-ui#4 represents the change I'd propose (I'll open a PR upstream with this diff later today).

Attached are two metafiles generated by esbuild with my MRE, to help visualize the bundle size changes with https://esbuild.github.io/analyze/

zod-mini.json
baseline.json

[louis-bompart]

Would also encompass & fix
#982 & #983

[ranst91]

As I mentioned in the PR, we're moving away from zod, therefore closing this issue as well.
Can be followed here #1637