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

[arvindKandpal-ksolves]

I've updated this PR based on the feedback:

• Cherry-picked the commit from master and amended the commit message to include the standard cherry-pick suffix.
• Ran manual tests on a local standalone cluster using the sanity script. When testing the bad syntax (list_namespace, 'ns.*') and variable assignments (list = 10), the shell correctly printed the expected WARN messages and JRuby SyntaxError.
• Most importantly, the shell didn't hang or break. All subsequent commands (create, put, scan, etc.) ran smoothly without corrupting the binding.
• Automated shell tests (mvn test -Dtest=TestShell -pl hbase-shell -Dhadoop.profile=3.0) also passed locally.

This looks ready from my side. Let me know if it's good to merge! I'll start working on the branch-2.5 backport next.

cc @NihalJain @Apache9 @liuxiaocs7 @vaijosh

[NihalJain]

Thank you @arvindKandpal-ksolves