C#: Add Preconditions.Not, And, and recipe-based Check support

[macsux]

Summary

• Add RecipeCheck, NotVisitor, and AndVisitor classes to support recipe-based precondition checks, negation, and AND composition in C#
• Add corresponding Preconditions.Check(Recipe, ...), Preconditions.Not(...), and Preconditions.And(...) static methods
• Widen SearchResult.Found<T> generic constraint from where T : J to where T : Tree to support non-Java tree types
• Add PreconditionsTest covering Check, Not, And, and recipe-based Check scenarios

Test plan

• New PreconditionsTest class with tests for all new precondition combinators
• dotnet build succeeds