Compatibility to Drupal 10.6.x no longer given

Problem/Motivation

The 2.0.x branch declares core_version_requirement: ^10.5 || ^11 in site_assistant.info.yml, but the code no longer works on Drupal 10.6.x. During the "prepare for Drupal 11" work the entity type definitions were converted from doctrine annotations to PHP attributes.
The Drupal\Core\Entity\Attribute\* attribute classes were only introduced in Drupal 11.1. They do not exist in Drupal 10.6.x. The regression was introduced by the late-October 2025 "prepare for Drupal 11" commit series on 2.0.x.

While checking the code, i also found another issue, were malformed route parameter placeholders were introduced in the links templates — the closing } was mistyped as ].

Steps to reproduce

• install Drupal 10.6.x.
• require and enable drupal/site_assistant:2.0.x-dev.
• entity types are not discovered; routes/collections/fields for the assistant entities are missing and the module is non-functional.

Proposed resolution

• revert the four entity types (Assistant, AssistantLibraryItem, AssistantListEntry, AssistantListEntryType) back to doctrine @ContentEntityType / @ConfigEntityType annotations, which work on both 10.6 and 11.x
• Fix the malformed route placeholders {…] → {…} in Assistant.php, AssistantLibraryItem.php, and AssistantListEntry.php

User interface changes

• none

API changes

• none

Data model changes

• none