test : added unit tests for CLI argument parsing and run_scan edge cases

[tmdeveloper007]

Closes #1227.

Summary of What Has Been Done:
Expanded testing/backend/unit/test_cli.py from 3 tests to 7, covering additional CLI edge cases not yet tested.

Changes Made:

• test_cli_no_args_calls_print_help: verifies main() with no args calls print_help without SystemExit
• test_run_scan_target_dot_defaults_to_secret_scanner: verifies target "." defaults to secret_scanner plugin
• test_run_scan_task_not_found_returns_1: verifies run_scan returns 1 when DB fetch returns None after task completion
• test_run_scan_failed_task_returns_1: verifies run_scan returns 1 when task status is failed without crashing on None structured_json
• 7 tests total, all passing

Impact it Made:
The CLI is the user-facing entry point. Unhandled edge cases (task disappearing, failed status) would cause tracebacks instead of clean exit codes. These tests ensure graceful degradation in all failure scenarios.

Note: This task is being handled by tmdeveloper007 — please assign to that account when picking it up.