[Improve] Remove SourceSplitEnumerator::currentUnassignedSplitSize

[HighandLight]

Purpose of this pull request

• Remove SourceSplitEnumerator::currentUnassignedSplitSize from the public API
• Simplify API surface by removing a test-only method
• Reduce maintenance since the method had no runtime usage

Does this PR introduce any user-facing change?

• No end-user behavior change
• For connector developers: This is a backwards-incompatible API change for enumerator implementations.
  If a custom enumerator previously overrode currentUnassignedSplitSize, remove that override and update dependent tests.

How was this patch tested?

• Updated unit tests to validate assignment outcomes (e.g., expected split count, no duplicates, balanced distribution) instead of calling currentUnassignedSplitSize().

• Verified via repository-wide search that no remaining references exist.

• Local quick build:

  ./mvnw -pl seatunnel-api,seatunnel-connectors-v2 -am \\
    -DskipITs -DskipIT -Dspotless.apply.skip=true -Dcheckstyle.skip=true \\
    -DskipDocker -DfailIfNoTests=false clean test

• Full CI will validate the entire reactor.

Changes overview

• API: Remove int currentUnassignedSplitSize() from org.apache.seatunnel.api.source.SourceSplitEnumerator
• Connectors: Remove overrides across all enumerators (e.g., JDBC, Kafka, Pulsar, ClickHouse, etc.)
• Tests: Replace direct assertions with assignment validations; update collections to standard Java types (avoid shaded Guava).

Check list

• No new Jar binary packages added.
• Documentation update not required for end users (API note covered here).
• Connector code updated as needed; no changes required to:
• plugin-mapping.properties
• seatunnel-dist/pom.xml
• .github/workflows/labeler/label-scope-conf.yml
• e2e tests
• config/plugin_config