[improve][misc] Add primitive Long2LongOpenHashMap

[void-ptr974]

Motivation

Part of #26027.

This adds a small primitive long-to-long hash map that can be reused by later PendingAcksMap storage changes. The patch is intentionally standalone and does not change broker behavior.

Modifications

• Add Long2LongMap as the minimal primitive map contract needed by the follow-up work.
• Add Long2LongOpenHashMap using open addressing with primitive long keys and values.
• Add oracle, random, collision, removal, zero/default-value, and edge-key/value coverage.

The map returns 0 for missing keys on get/put/remove, matching the existing primitive-map style in this package. Callers that need to distinguish a stored zero value can use containsKey or getOrDefault.

Verifying this change

• ./gradlew :pulsar-common:test --tests org.apache.pulsar.common.util.collections.Long2LongOpenHashMapTest --max-workers=1 -PtestRetryCount=0
• ./gradlew :pulsar-common:checkstyleMain :pulsar-common:checkstyleTest --max-workers=1

[lhotari]

I'd rather use Fastutil for this purpose. However, it was dropped in #25413 due to a problem in implementing a gradle based solution to pick classes from Fastutil that are needed instead of pulling in the whole fastutil dependency to Pulsar client. In the maven build pulsar-client-dependencies-minimized handles including the classes which are needed.
The fastutil jar is over 25MB is size and that's why a minimizer solution is needed.
I'll take a look if we can revert #25413 and go back to a solution where it's minimized with Gradle. The reason for this is that there have been bugs in the past in custom collection implementations and it's a maintenance overhead for the project to implement various primitive collections to address different use cases (where keys and values are of different type).
I'll block this PR until I have checked the possibility of reverting #25413 and re-implementing pulsar-client-dependencies-minimized with Gradle. If that's not possible, let's go ahead with this PR.

[lhotari]

PR to restore fastutil is #26032