Add Bazel build targets for Python parser

[ishijo]

Summary

Adds Bazel build targets for the Python language parser and its tree-sitter base to Vanir's build system.

Objective

The PythonParser implementation requires two new Bazel build targets:

• tree_sitter_base: wraps the shared TreeSitterParserBase class used by tree-sitter-backed parsers
• python_parser: builds the Python parser and tells Bazel it needs the tree-sitter and tree-sitter-python pip packages to compile

The language_parsers aggregator target is updated to include python_parser so the dispatcher automatically discovers and uses it for .py files.

Changes

vanir/language_parsers/BUILD.bazel

• Added tree_sitter_base build target
• Added //vanir/language_parsers/python:python_parser to language_parsers deps

vanir/language_parsers/python/BUILD.bazel (new file)

• Added python_parser build target with tree-sitter and tree-sitter-python declared as pip dependencies
• Added python_parser_test build target for the corresponding unit tests

Dependencies

This PR is independent and can be merged in any order alongside:

• feat/python-parser-deps
• feat/tree-sitter-integration
• bug/fix-subclass-discovery

Must be merged before feat/python-parser.