Fix potential crashes and unhandled exceptions in spark-intelligence-builder helpers (Batch 114)

[Ra229287]

This PR fixes bugs #3885 to #3889.

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    "title": "Potential unhandled crashes in spark-intelligence-builder helpers (Batch 114)",
    "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 _safe_mtime in src/spark_intelligence/swarm_bridge/local.py",
      "Pass invalid types to _resolve_latest_round_summary_path in src/spark_intelligence/swarm_bridge/local.py",
      "Pass invalid types to _round_history_path in src/spark_intelligence/swarm_bridge/local.py",
      "Pass invalid types to _load_round_history in src/spark_intelligence/swarm_bridge/local.py",
      "Pass invalid types to _load_json_file in src/spark_intelligence/swarm_bridge/local.py"
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    "affected_workflow": "spark-intelligence-builder 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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