[codex] Remove eval from schema parser

[anansutiawan]

Summary

• replace the final schema parser eval(compile(...)) step with a recursive AST constructor
• keep the existing validated grammar for constants, validator-name references, and nested validator calls
• add parser regression coverage for nested validator-name references and unary numeric constants

Rationale

PR #173 added AST validation as a best-effort hardening step and noted that removing eval entirely would be the stronger follow-up. This keeps the same schema syntax while avoiding runtime evaluation of schema expressions.

Validation

• pytest yamale (169 passed)
• ruff check yamale/syntax/parser.py yamale/syntax/tests/test_parser.py
• bandit yamale/syntax/parser.py -q

[anansutiawan]

Closing this draft and reopening from a cleaner branch and title.