Fix potential crashes and unhandled exceptions in spark-researcher helpers (Batch 97)

[Ra229287]

This PR fixes bugs #2695 to #2699.

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    "title": "Potential unhandled crashes in spark-researcher helpers (Batch 97)",
    "actual_behavior": "Various helper functions in spark-researcher 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 _infer_document_kind in src/spark_researcher/memory.py",
      "Pass invalid types to _manifest_docs_by_path in src/spark_researcher/memory.py",
      "Pass invalid types to _local_search_results in src/spark_researcher/memory.py",
      "Pass invalid types to _build_snippet in src/spark_researcher/memory.py",
      "Pass invalid types to build_run_doc in src/spark_researcher/memory.py"
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    "affected_workflow": "spark-researcher utility 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 helper functions.",
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