Fix potential crashes and unhandled exceptions in subsystem helpers (Batch ext-45)

[Ra229287]

This PR fixes bugs #1120 to #1124.

Packet

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    "title": "Potential unhandled crashes in subsystem helpers (Batch ext-45)",
    "actual_behavior": "Various subsystem helper functions crashed with raw tracebacks when variables did not conform to expected formats.",
    "expected_behavior": "All helper functions should gracefully check parameter structures and parameter types, falling back to safe defaults without tracebacks.",
    "repro_steps": [
      "Pass invalid types to ssh_host_alias in src/spark_cli/sandbox/ssh.py",
      "Pass invalid types to ssh_keyscan_argv in src/spark_cli/sandbox/ssh.py",
      "Pass invalid types to parse_ssh_keyscan_output in src/spark_cli/sandbox/ssh.py",
      "Pass invalid types to fingerprint_known_host_line in src/spark_cli/sandbox/ssh.py",
      "Pass invalid types to scan_ssh_host_key in src/spark_cli/sandbox/ssh.py"
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    "affected_workflow": "CLI Subsystem lifecycle"
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    "before_after_proof": "Before: raw traceback crashes. After: safe type validation guards prevent all tracebacks.",
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  "proposed_fix": {
    "approach": "Added explicit type checks, try-except containment guards, and fallback initializers across 5 subsystem helper functions.",
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