Upgrade Web SDK to React 19 and ReScript 12

[ArushKapoorJuspay]

Summary

Modernize the HyperSwitch Web SDK build stack to React 19 and ReScript 12 while preserving existing SDK behavior and merchant-facing APIs.

Goals

• Upgrade React and React DOM to 19.2.8.
• Upgrade ReScript to 12.3.0.
• Upgrade @rescript/react to 0.15.0.
• Add @rescript/runtime 12.3.0.
• Preserve classic JSX mode and Jotai store isolation.
• Maintain existing v1 and v2 SDK behavior.
• Update the React demo and wrapper integration.
• Preserve existing Promise rejection semantics.

Acceptance Criteria

• ReScript compilation succeeds.
• Default v1 and v2 production builds succeed.
• Existing lint checks pass.
• React demo v1 and v2 builds succeed.
• Dependency trees contain no invalid peer dependencies.
• Promise rejection handling supports null, undefined, JavaScript values, native errors, JsExn, and ReScript exceptions.
• Existing merchant-facing APIs remain unchanged.