Update OAuth application

Rate limit: 60 requests per 60 seconds. This is the default shared quota — it is shared with every other endpoint that has no dedicated limit, so requests across those endpoints all draw from the same budget.

Updates one or more modifiable fields of an existing OAuth application. Only the application's admin (the caller's GCID must be in adminGcids) can perform updates. Non-admins receive 404 Not Found (opaque authorization - the gateway does not disclose whether the application exists). At least one field must be provided in the body; omitted fields are left unchanged. Uniqueness and length constraints match the create endpoint.