Add type annotations to vyper parse.py internals

[augmentcode]

Summary

• add explicit annotations for previously untyped local variables in vyper/vyper/ast/parse.py
• add return and parameter annotations for internal helper functions and visitor methods in the same file
• keep the change behavior-preserving, with only minor type-safety assertions around existing assumptions

Validation

• python -m py_compile vyper/ast/parse.py
• structural AST check confirming there are no remaining untyped defs in vyper/vyper/ast/parse.py

Notes

• python -m pytest tests/unit/ast/test_parser.py tests/unit/ast/test_tokenizer.py tests/unit/ast/test_annotate_and_optimize_ast.py could not run because pytest is not installed in the current environment
• direct runtime smoke checks could not run because the current environment is missing the Crypto dependency required by Vyper imports
• manual-session GitHub attribution is misconfigured here: github-api was unavailable, so this PR was created with github-app-api instead

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[augmentcode]

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[augmentcode]

The new return annotation -> python_ast.For is inconsistent with the function's actual return: return self.generic_visit(node) on the last line returns python_ast.AST per the new generic_visit annotation on line 211, so mypy will flag this as an incompatible return type. Consider parameterizing generic_visit with a TypeVar (e.g. _AstT) so subclass methods can return their narrow types, or cast(...) the result here.

Severity: low

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[augmentcode]

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Fixed in c6d510a by parameterizing generic_visit with _AstT and casting the super().generic_visit(...) result back to that type. Validation: python -m py_compile vyper/vyper/ast/parse.py; structural annotation check passed. (python -m mypy ... is unavailable here because mypy is not installed.)