Fix potential crashes and unhandled exceptions in spark-intelligence-builder helpers (Batch 118)

[Ra229287]

This PR fixes bugs #3905 to #3909.

Packet

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    "title": "Potential unhandled crashes in spark-intelligence-builder helpers (Batch 118)",
    "actual_behavior": "Various helper functions in spark-intelligence-builder 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 _build_hook_import_summary in src/spark_intelligence/attachments/snapshot.py",
      "Pass invalid types to _require_known_key in src/spark_intelligence/attachments/snapshot.py",
      "Pass invalid types to _get_string_list in src/spark_intelligence/attachments/snapshot.py",
      "Pass invalid types to _existing_keys in src/spark_intelligence/attachments/snapshot.py",
      "Pass invalid types to _normalize_optional_string in src/spark_intelligence/attachments/snapshot.py"
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    "affected_workflow": "spark-intelligence-builder utility lifecycle"
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  "evidence": {
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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 helper functions.",
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