chore(rust/signed-doc): Update Validator::validate return type

[Mr-Leshiy]

Description

Modify Validator::validate return type.
Do not return boolean flag which is a duplication of the filling doc.report().

Related Issue(s)

Part of #634

Description of Changes

Provide a clear and concise description of what the pull request changes.

Breaking Changes

• Validator::validate return type. Instead of relying on the returned boolean flag, use doc.report().is_problematic()

Please confirm the following checks

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module

[github-actions]

📚 Docs Preview

The docs for this PR can be previewed at the following URL:

https://docs.dev.projectcatalyst.io/libs/chore/validate-interface

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 715/715}$ | ${\color{red}Fail: 0/715}$ |