refactor(kafka_topic): migrate KafkaTopic to Reconciler[T]

[ndk]

About this change—what it does

Resolves: #NEX-2284.

Migrates KafkaTopic to the shared generic Reconciler[T] and adds scenario tests to lock in behavior.

What stays the same:

• Preconditions: the Kafka service must exist and have enough RUNNING nodes for the requested replication factor.
• Create/Update payload and mapping (partitions/replication/tags/config).
• READY semantics: we still mark the resource running once the topic becomes ACTIVE (instance-is-running annotation + Running condition).
• Delete semantics: respects termination_protection; NotFound on delete is treated as success.
• Singleflight is still used to dedupe ServiceKafkaTopicList calls per service.

Behavior changes / important notes:

• Continuous reconciliation: the controller now keeps polling even after the resource becomes READY.
• Existence drift: if the Kafka topic is deleted on the Aiven side while the Kubernetes KafkaTopic resource still exists, the operator will re-create it.
• Transient 5xx from ServiceKafkaTopicList are treated as retryable without treating the topic as missing. During spec updates we assume the topic exists if it was applied before to avoid going through Create and hitting AlreadyExists.

Out of scope:

• Config drift correction (spec vs remote config) is intentionally not implemented here. This PR only adds existence drift detection.

Tests:

• Adds scenario tests covering preconditions, create/update, topic state transitions, transient 5xx handling, deletion/termination protection, and existence drift re-creation.

[ndk]

/test

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[vmyroslav]

I would add to the change log the detailed explanation of changes which might be breaking for someone. Like the fact that now operator WILL re-create the deleted topics.

[ndk]

/test

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