Feature: Add AST-based unused variable detection

[himnishag]

Description

This PR adds AST-based unused variable detection to the Python analyzer.

What was added

• Implemented detect_unused_variables() using Python's built-in AST module.
• Detects variables that are assigned but never used later in the code.
• Ignores intentionally unused variables prefixed with _.
• Integrated the new detection into the main analyze() pipeline.
• Added unit tests to verify detection and prevent false positives.

Example

x = 10
y = 20
print(x)

Detected issue:

• y is assigned but never used.

Related Issue

Fixes # 45

Type of change

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

Checklist

• I have read 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

Test evidence

python -m pytest tests/test_ast_analyzer.py

============================= test session starts =============================
collected 43 items

tests/test_ast_analyzer.py ........................................... [100%]

============================= 43 passed in 0.18s =============================

[imDarshanGK]

@himnishag issue number?

[himnishag]

issue number is

Fixes #791

[himnishag]

@imDarshanGK

I have fixed the lint and formatting issues and pushed the latest changes.

Could you please approve the pending workflows and review the PR?

Thank you.

[imDarshanGK]

@himnishag

[himnishag]

@imDarshanGK

I have fixed the formatting issue and pushed the latest commit (Apply CI black formatting).

The workflows are currently awaiting maintainer approval. Could you please approve the pending workflows and re-review the PR?

Thank you.

[imDarshanGK]

@himnishag resolve conflicts

[himnishag]

Hi @imDarshanGK,

I've resolved the merge conflicts in the PR and merged the latest main into my branch. The conflicting files (main.py, database.py, and test_history.py) have been resolved and pushed.

Could you please review it again? Let me know if any further changes are needed.

Thanks!