Fix potential crashes and unhandled exceptions in subsystem helpers (Batch ext-42)

[Ra229287]

This PR fixes bugs #1105 to #1109.

Packet

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    "title": "Potential unhandled crashes in subsystem helpers (Batch ext-42)",
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    "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 _parse_ipv4_number in src/spark_cli/sandbox/ssh.py",
      "Pass invalid types to _legacy_ipv4_address in src/spark_cli/sandbox/ssh.py",
      "Pass invalid types to _ssh_host_ip in src/spark_cli/sandbox/ssh.py",
      "Pass invalid types to _is_metadata_host in src/spark_cli/sandbox/ssh.py",
      "Pass invalid types to validate_ssh_host in src/spark_cli/sandbox/ssh.py"
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    "affected_workflow": "CLI Subsystem lifecycle"
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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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