fix(inkless:switch): update UnderReplicatedPartition for switched partitions

[giuseppelillo]

No description provided.

[Copilot]

Pull request overview

This PR fixes an operational issue where ReplicaManager’s UnderReplicatedPartitions aggregate could remain non-zero (or otherwise not reflect live ISR state) after switching a topic from classic to diskless, by ensuring the leader continues to observe the follower’s fetch state until the replica is back in ISR.

Changes:

• Update follower fetch routing to treat fetchOffset == classicToDisklessStartOffset as still eligible for unified-log reads for follower requests, enabling the leader to observe fetch progress at the seal offset.
• Ensure diskless followers schedule a fetch even when caught up to the seal but currently out of ISR, so the leader can expand ISR again.
• Add a system test that stops a follower after switch, asserts URP rises, then restarts the follower and asserts URP clears after ISR fully recovers.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/kafkatest/tests/inkless/inkless_topic_switch_test.py	Adds URP JMX scraping + a new test to verify URP rises on replica loss and clears after replica recovery post-switch.
core/src/main/scala/kafka/server/ReplicaManager.scala	Adjusts diskless follower fetch eligibility at the seal offset and ensures fetching is scheduled when out of ISR even if already caught up.
core/src/main/scala/kafka/server/ReplicaFetcherThread.scala	Prevents evicting switched partitions from the replica fetcher until the replica is back in ISR.