HBASE-28660 list_namespace not working after an incorrect user input (branch-2)

[arvindKandpal-ksolves]

Why are the changes needed?

This is a backport of the fix for HBASE-28660 to branch-2.
When a user enters an incorrect command like list_namespace, 'ns.*', the JRuby parser mistakenly creates a local variable initialized to nil before throwing a SyntaxError. Because Ruby prioritizes local variables over method calls, subsequent valid commands return empty/nil instead of executing the actual shell command.

How does this PR fix the problem?

• Added the same cleanup mechanism inside hirb.rb's eval_input loop that was merged into the master branch.
• Checks for collisions (shadowing) with registered HBase shell commands.
• Warns the user via stderr and safely discards the polluted binding.
• Includes the updated comprehensive test cases in general_test_cluster.rb covering all 5 affected commands (list, list_namespace, list_snapshots, scan, processlist).

Tests run

Ran the shell tests locally on branch-2 with the Hadoop 3 profile and verified that all tests pass without regressions.
mvn test -Dtest=TestShell -pl hbase-shell -Dhadoop.profile=3.0