[AI Bundle] Simplify memory configuration structure

[OskarStark]

Q	A
Bug fix?	no
New feature?	yes
Docs?	yes
Issues	Fix #591
License	MIT

Remove guessing logic for memory keys and introduce explicit syntax:

memory: 'text' # always creates static memory processor

memory:
    service: 'service_name' # uses service reference

This makes the configuration more predictable and eliminates the need to check if a string refers to an existing service.

cc @DZunke

[chr-hertel]

Thanks @OskarStark for the quick solution - will directly use that in my Dresden workshop 😎