feat: expose prepared FormRequest data during validation failure

[memleakd]

Description

This PR proposes making the prepared FormRequest payload available while handling validation failure.

When a FormRequest normalizes input in prepareForValidation(), that prepared data is what the validator actually checks. But if validation fails, failedValidation() did not have a clean way to access that same payload. The user guide example for flashing normalized old input had to call prepareForValidation($this->validationData()) a second time just to rebuild it, which felt a bit off.

This PR stores the prepared validation data when the FormRequest is resolved, and exposes it to subclasses with getPreparedValidationData(). That gives custom failedValidation() implementations access to the exact payload that was passed to validation, without running preparation twice.

The main use case is small and practical: if a request trims or canonicalizes visible form fields, a custom failure response can flash those prepared values back to the form instead of flashing raw submitted input.

This does not change the default validation failure response. Also, the prepared payload is still not trusted data; successful requests should continue using getValidated() or getValidatedInput().

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide