⚡ Bolt: [performance improvement] Replace nested ast.walk with ast.NodeVisitor

[ishaanxgupta]

💡 What: Replaced nested ast.walk() loops in PythonParser._extract_calls, _compute_complexity, and _extract_imports with ast.NodeVisitor implementations.

🎯 Why: The nested ast.walk() loops caused O(N^2) time complexity during AST traversal because every node traversal generated all its sub-nodes iteratively, leading to heavy redundant processing for deeply nested nodes. This slowed down the code scanner significantly on large Python files.

📊 Impact: Reduces AST parsing time complexity for these operations from O(N^2) to O(N), substantially improving indexing speed for large source code files without changing behavior or sacrificing readability.

🔬 Measurement: Verify tests pass, and observe exact indexing time during full repository parsing using performance profiling tools. The change is isolated and deterministic.

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PR created automatically by Jules for task 7808889064143730086 started by @ishaanxgupta

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