refactor: update open api client generation, update docs

Author: hobbescodes

.changeset/openapi-async-client.md

@@ -0,0 +1,7 @@
+---
+"tangrams": patch
+---
+
+Change OpenAPI client from static `$fetch` instance to async `getClient()` function pattern, matching the GraphQL client approach. This allows users to add dynamic headers (e.g., auth tokens) that require async operations.
+
+**Breaking Change:** If you have customized your OpenAPI `client.ts` file, you will need to migrate from the `$fetch` export to the new `getClient()` async function pattern.