YQL-21134: Set UnstableFormat AST flag unconditionally

- Renamed `TAstNodeFlags::IgnoredContent` to `TAstNodeFlags::UnstableFormat` to better reflect its purpose.
- Removed the `MarkQueryTextAtomWithIgnoredContent` translation setting. The `UnstableFormat` flag is now unconditionally applied to query text atoms during parsing.
- Updated `CheckedFormat` and `ValidateAST` to accept an already parsed AST. Since the flag is now set unconditionally, we can reuse the initial AST for format validation. This avoids re-parsing the original query, reducing redundant AST builds and overall RAM consumption.
commit_hash:bba3e6733d3a9c120cf9724a161e603de0cfc038

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