[AAP-78702] Add CleanTextMixin to serializers

[daphnemaeve]

Jira ticket: https://redhat.atlassian.net/browse/AAP-78702

Applies CleanTextMixin (from django-ansible-base) to EDA API serializers so that free-text input (name, description, and similar CharField/TextField values) is validated against the shared allowlist/blocklist rules before being persisted

Wires the mixin into the write-path serializers across activation, credential_input_source, credential_type, decision_environment, eda_credential, event_stream, organization, project, team, and user. Each validate() override now chains to super().validate(...) so the mixin's checks actually run. Fields that legitimately contain non-plain-text content (e.g. extra_var/injectors Jinja2 templates, inputs/metadata credential payloads, token secrets) are excluded via excluded_fields so they aren't rejected as unsafe.

ATF tests: https://gitlab.cee.redhat.com/ansible/testing/eda-test-suite/-/merge_requests/142

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