fix(api-reference): convert flow-style YAML to JSON on download - #10545

The public downloadDocument helper treated any input beginning with { or [ as JSON. Flow-style YAML could therefore be downloaded unchanged with a .json extension and application/json MIME type.

For example, this complete OpenAPI document is valid YAML:

{openapi: 3.1.0, info: {title: Test, version: 1.0.0}, paths: {}}

import { downloadDocument } from '@scalar/api-reference/helpers'

await downloadDocument(
  '{openapi: 3.1.0, info: {title: Test, version: 1.0.0}, paths: {}}',
  'openapi',
  'json',
)

Before this change, the downloaded content remained YAML and failed JSON.parse. The helper now verifies JSON by parsing it, so flow-style YAML is converted to valid JSON when JSON is requested. Downloads that need no format conversion preserve their original content.

Ticket

Fixes #10541