source-hubspot: custom object stream names can collide with built-in streams

## Problem

HubSpot custom CRM objects can be created with a `name` that matches one of the Airbyte source-hubspot built-in streams.

For example, a HubSpot custom object named `form_submissions` collides with the built-in Airbyte `form_submissions` stream. During discovery, Airbyte can then produce two catalog streams with the same `(name, namespace)` pair:

- `name=form_submissions`
- `namespace=null`

This makes the catalog ambiguous and can prevent connection creation/update because the backend cannot uniquely identify the stream.

## Expected behavior

Custom object streams should be discovered with a unique, portal-qualified name so they cannot collide with built-in streams.

HubSpot schemas expose `fullyQualifiedName` values such as `p123456_form_submissions`, which are already suitable for uniquely identifying custom objects.

## Proposed fix

Use `fullyQualifiedName` instead of the non-unique custom object `name` when mapping names for dynamic custom object streams in `source-hubspot`.

## Impact

This should unblock users whose HubSpot custom object names collide with built-in connector stream names. Users already syncing custom object streams may need to refresh schema and reselect the renamed custom object stream.