KAFKA-20692: Add unit tests for ToolsUtils

[Dpk376]

Summary

`ToolsUtils` in the `tools` module contains five public methods (`printMetrics`, `prettyPrintTable`, `validateBootstrapServer`, `duplicates`, `minus`) with no corresponding unit tests. This PR adds `ToolsUtilsTest.java` covering:

• `validateBootstrapServer` — valid single host, valid CSV hosts, null/empty/blank/no-port/mixed-invalid all throw `IllegalArgumentException`
• `duplicates` — no dupes, single dup, multiple dups, empty list, all-same
• `minus` — removes present elements, ignores absent, empty set, no-args copy, does not mutate original
• `prettyPrintTable` — headers present in output, rows present, column widths aligned to max value length

All tests are pure unit tests (no cluster required).

JIRA

https://issues.apache.org/jira/browse/KAFKA-20692

[Dpk376]

@chia7712 when you have a moment — KAFKA-20692 adds unit tests for ToolsUtils. It looks like the CI workflow is awaiting approval to run; would you mind approving it and taking a look? Thanks!