YQL-21315: Implement variant expensive validation

#### Implement expensive datum validation for variant types ✎

- Introduce comprehensive validation for variant data types that checks offset invariants to ensure data integrity
- Add support for passing validation mode through fuzzer infrastructure to enable both cheap and expensive validation paths
- Extend test helpers to support different validation modes and integrate validation into various test cases
- Modify datum validation logic to differentiate between cheap and expensive validation modes, with expensive mode performing additional checks on dense union structures
- Update fuzzer components to validate data at multiple points during transformation to catch potential issues early

<a href="https://nda.ya.ru/t/qa0kX64r7DqvtN"><font size="2">Autodescription by Yandex Code Assistant</font></a>
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