feat: add sensitive struct

[ccharly]

Add a new struct sensitive that can used with object or type. Any value for this field will get redacted automatically if any validation/errors happen.

This also apply if a sibling field validation is failing, so there's no way to make the value escape out of the actual payload being validated.

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Note

Medium Risk
Changes the validation error surface for object schemas and failure messages, which is security-relevant for secret handling but is additive and covered by extensive validation tests.

Overview
Adds a sensitive() wrapper so validation failures replace the real value with *** in StructError messages, value, and branch, including refiner failures where the default message would otherwise echo the secret.

object() and type() now detect branded sensitive fields and wrap sibling entry structs so when another key fails, the parent object in branch is shallow-copied with sensitive keys redacted (using context.branch so this still works under coercion). Struct copies Symbol-keyed brands when wrappers spread an inner struct, so optional / exactOptional compositions keep the sensitive marker without extra changes.

Also exports isSensitiveStruct and documents the feature in the changelog.

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[ccharly]

This is necessary to forward the brand (marker) inside structs when we are re-constructing them.

Type-wise, that does not change anything, those are just plain symbols that are used as markers for us to detect later if a re-constructed struct was banded as sensitive (we could use this same logic for other kind of decorators in the future too).

[hmalik88]

LGTM, but want @Mrtenz to take a final look 🙇‍♂️