feat: add global variable symbols to workspace search

[harshatba]

Summary

Extends symbol indexing to include module-level variable definitions, making them searchable via workspace symbol search (Cmd+T / Ctrl+T).

Changes

• ✅ Add Variable variant to SymbolKind enum
• ✅ Update LSP handler to map Variable to LspSymbolKind::VARIABLE
• ✅ Implement assignment extraction in tree-sitter parser
  • Simple assignments: MY_VAR = 42
  • Annotated assignments: name: str = "hello", server: str
  • Tuple unpacking: x, y = 1, 2
  • Chained assignments: a = b = 10
• ✅ Implement assignment extraction in Ruff parser with same support
• ✅ Add comprehensive unit and integration tests (both parser backends)
• ✅ Create test fixture with positive and negative test cases

Behavior

• Included: Module-level (global) variables only
• Excluded: Local variables in functions, class attributes (to avoid cluttering search results)

Test Plan

All 57 tests pass:

• 28 unit tests (including 7 new variable-specific tests)
• 27 integration tests (including new global variables test)
• 2 parallel indexing tests

Both parser backends (tree-sitter and ruff) produce matching results.

Testing Locally

1. Build: cd pylight && cargo build --release
2. Copy binary: cp pylight/target/release/pylight pydance/
3. Package extension: cd pydance && npx @vscode/vsce package
4. Install: code --install-extension pydance-*.vsix
5. Test with a Python file containing global variables

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