Audit and apply `pub(crate)` visibility to internal `validate_*` helper functions

[elycruz]

Summary

Several validate_* helper functions (e.g., validate_str, validate_len, validate_scalar, validate_step, etc.) in the walrs_validation crate are currently pub fn. If these are internal implementation details and not part of the public API, they should be scoped to pub(crate) to reduce the public surface area and prevent unintended external usage.

Affected Files

• crates/validation/src/rule_impls/string.rs — validate_str, validate_str_all, validate_str_option, validate_str_option_all
• crates/validation/src/rule_impls/length.rs — validate_len, validate_len_all, validate_option_len
• crates/validation/src/rule_impls/scalar.rs — validate_scalar, validate_scalar_all, validate_scalar_option, validate_scalar_option_all
• crates/validation/src/rule_impls/steppable.rs — validate_step, validate_step_all, validate_option_step, validate_option_step_all

Tasks

• Audit each pub fn validate_* function to determine whether it is used outside the crate.
• Change visibility from pub fn to pub(crate) fn for functions that are only used internally.
• Verify the crate and dependent crates still compile after changes.