feat: build AST-based Python analyzer using built-in ast module (closes #169)

[antharya05]

Description

Implemented an AST-based Python analyzer using Python’s built-in ast module for deeper and more accurate bug detection beyond the existing regex-based approach.

Changes made

• Added backend/app/services/ast_analyzer.py

• Implemented PythonASTAnalyzer using ast.NodeVisitor

• Added AST-based detections for:

  • mutable default arguments
  • bare except:
  • eval() / exec() usage
  • built-in shadowing
  • unreachable code after return / raise

• Added graceful syntax error handling

• Integrated AST analysis into run_bug_detection() for Python

• Preserved existing regex pipeline for compatibility

• Added comprehensive AST analyzer test suite

• Added rate-limit reset fixture to prevent test leakage between endpoint tests

Related Issue

Fixes #169

Type of change

• Bug fix
• New feature / enhancement
• Documentation update
• Test addition
• Refactor

Checklist

• I have read [CONTRIBUTING.md](../CONTRIBUTING.md)
• My branch is up to date with main
• I have run pytest -v and all tests pass
• I have not introduced duplicate issues or features
• My PR title follows the format: feat/fix/docs/test: short description
• I have added tests for new features (Level 2 and 3 issues)
• No hardcoded secrets or API keys in my code
• This PR is linked to a GSSoC 2026 issue

Screenshots (if frontend change)

N/A — backend feature only.