CASSANDRA-21548: cqlsh prompt does not reset to default after dropping the current keyspace

[arvindKandpal-ksolves]

Summary

This PR fixes an issue where the cqlsh shell prompt remains stuck displaying a keyspace name even after that keyspace has been successfully dropped.

Problem

When a user is currently inside a keyspace in cqlsh (e.g., cqlsh:test_ks>) and executes DROP KEYSPACE test_ks;, the server deletes the keyspace, but the client-side cqlsh prompt does not reset to the default cqlsh> prompt. Executing subsequent queries inside the dropped keyspace context throws a Keyspace does not exist error.

Solution

• Updated perform_simple_statement in pylib/cqlshlib/cqlshmain.py to check if an executed statement is a DROP KEYSPACE statement.
• If the dropped keyspace matches self.current_keyspace, self.current_keyspace is reset to None, which automatically updates the shell prompt back to cqlsh>.
• Included safe string manipulation and .lower() normalization to handle edge cases like quoted keyspace names ("Test_KS") and IF EXISTS clauses.

Testing

• Added automated unit tests in pylib/cqlshlib/test/test_cqlsh_output.py covering:
  1. Standard keyspace drop resetting the prompt.
  2. Dropping a non-active keyspace without altering the current prompt.
  3. Case-sensitive quoted keyspace names and DROP KEYSPACE IF EXISTS syntax.
• Verified test execution using pytest (python3 -m pytest pylib/cqlshlib/test/test_cqlsh_output.py -k "test_drop_keyspace_prompt_behavior").

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patch by Arvind Kandpal; reviewed by TBA for CASSANDRA-21548