Fix TypeScript error in processPromise return type (#2204)

## Summary
- Fixed TypeScript compilation error in `serverRenderReactComponent.ts`
that was blocking the release script
- The error occurred because TypeScript couldn't narrow the type
properly after the `isValidElement` check in the `.then()` callback
- Added explicit type assertion to `FinalHtmlResult` since after the
`isValidElement` check, the remaining types (`string` and
`ServerRenderHashRenderedHtml`) are all valid `FinalHtmlResult` types

## Test plan
- [x] TypeScript type-check passes (`pnpm run type-check`)
- [x] Build completes successfully (`pnpm run build`)
- [x] ESLint passes (`pnpm run lint`)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Refactor**
* Improved type safety for server-side rendering operations to ensure
more robust handling of promise resolutions.

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settings.</sub>

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Claude <noreply@anthropic.com>

Author: justin808

packages/react-on-rails/src/serverRenderReactComponent.ts

@@ -9,6 +9,7 @@ import { renderToString } from './ReactDOMServer.cts';
 import { createResultObject, convertToError, validateComponent } from './serverRenderUtils.ts';
 import type {
   CreateReactOutputResult,
+  FinalHtmlResult,
   RenderParams,
   RenderResult,
   RenderState,
@@ -71,7 +72,8 @@ function processPromise(
     if (isValidElement(promiseResult)) {
       return processReactElement(promiseResult);
     }
-    return promiseResult;
+    // promiseResult is string | ServerRenderHashRenderedHtml (both are FinalHtmlResult)
+    return promiseResult as FinalHtmlResult;
   });
 }