Respect false values for the validation error raise flag

[xiehuanyi]

Passing r_ve=False currently enables raising because the decorator checks whether the keyword is present, rather than its value. This contradicts the documented switch and breaks callers that pass a boolean variable to choose between returned and raised validation errors.

Use the flag value while retaining the environment override. Tests exercise both a validator that returns false (slug) and one that raises while parsing (uuid), and verify that explicit true and RAISE_VALIDATION_ERROR=True continue to raise. Before the change, the two false-flag regressions fail. Afterwards the full suite passes: 1,177 tests and doctests, with complete Ruff lint/format, strict Pyright and Bandit checks also passing.

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