fix: type augmentation of RequestContext (#9035)

### What?

Makes this to actually work
```ts
import type { RequestContext as OriginalRequestContext } from 'payload'

declare module 'payload' {
  // Create a new interface that merges your additional fields with the original one
  export interface RequestContext extends OriginalRequestContext {
    myObject?: string
    // ...
  }
}
```
<img width="502" alt="image"
src="https://github.com/user-attachments/assets/38570d3c-e8a8-48aa-a57d-6d11e79394f5">


### Why?
This is described in our docs
https://payloadcms.com/docs/beta/hooks/context#typescript therefore it
should work.

### How?
In order to get the declaration work, we need to reuse the type from the
root file `payload/src/index.js`. Additionally, removes `RequestContext`
type duplication in both `payload/src/types/index.js` and
`payload/src/index.js`.

Fixes https://github.com/payloadcms/payload/issues/8851

test/_community/int.spec.ts

@@ -54,6 +54,7 @@ describe('_Community Tests', () => {
       data: {
         text: 'LOCAL API EXAMPLE',
       },
+      context: {},
     })
 
     expect(newPost.text).toEqual('LOCAL API EXAMPLE')

test/_community/types.d.ts

@@ -0,0 +1,9 @@
+import type { RequestContext as OriginalRequestContext } from 'payload'
+
+declare module 'payload' {
+  // Create a new interface that merges your additional fields with the original one
+  export interface RequestContext extends OriginalRequestContext {
+    myObject?: string
+    // ...
+  }
+}