Add test fixtures and unit tests for each language extractor

[DhanushNehru]

The extraction engine currently has no test fixtures. Add small sample files for each supported language and verify that extraction produces the expected nodes and edges.

What needs to change:

Create a tests/fixtures/ directory with sample files:

• tests/fixtures/sample.py — a Python file with a class, function, import, and function call
• tests/fixtures/sample.js — same for JavaScript
• tests/fixtures/sample.rs — same for Rust
  (one file per supported language)

Add integration tests in tests/extract_test.rs that:

• Parse each fixture file using extract_file()
• Assert the correct number of nodes and edges are extracted
• Assert specific node kinds (e.g., the Python file should have a NodeKind::Class and a NodeKind::Function)
• Assert import edges point to the correct target

Keep fixtures minimal — 10-20 lines each, just enough to exercise all extraction features