CR-006: Add Oracle and TSQL national string literal support for N'...'

[yigalrozenberg]

Summary

Adds full support for national string literals with the N prefix across tokenization, parsing, AST, generation, and tests.

Changes

• Add TokenType::NationalString
• Tokenize N'...' and n'...' as national string tokens
• Add Expr::NationalStringLiteral
• Parse national string tokens into dedicated AST nodes
• Emit N'...' for Oracle/TSQL and plain strings for other dialects
• Update optimizer, type annotation, executor, and dialect transform handling
• Add tokenizer and transpile tests for roundtrip and cross-dialect behavior
• Update CLI cast expectation to current Postgres output style

Validation

• cargo test -q passes

Related

• CR-006
• PSQ-2379
• PSQ-1473