chore(docs): use renderedFieldDescriptions for single as well as multi requests (#5939)

## Short description of the changes made

Add `renderedFieldDescriptions` prop to single-request `EndpointRequestSection` usage, matching the existing multiple-requests implementation.

## What was the motivation & context behind this PR?

Found inconsistency during migration audit: multiple-request endpoints passed `renderedFieldDescriptions` for form data MDX rendering, but single-request endpoints did not. This meant form data field descriptions (for `file`, `files`, `property` fields) were rendered as plain text instead of MDX for single-request endpoints.

## How has this PR been tested?

- Verified no TypeScript/lint errors
- Confirmed fix matches existing pattern used in multiple-requests case

Author: stephencme

packages/fern-docs/bundle/src/components/api-reference/endpoints/EndpointContentLeft.tsx

@@ -216,6 +216,10 @@ export async function EndpointContentLeft({
                                     request={endpoint.requests[0]}
                                     types={types}
                                     lang={lang}
+                                    renderedFieldDescriptions={renderFormDataFieldDescriptions(
+                                        endpoint.requests[0],
+                                        types
+                                    )}
                                     PropertyRenderer={PropertyRenderer}
                                     PropertyWithShape={PropertyWithShape}
                                     TypeReferenceDefinitions={TypeReferenceDefinitions}