Add Racket tree-sitter support, remove Kotlin

- Add tree-sitter-racket dependency (v0.24)
- Initialize Racket parser in code search
- Add .rkt, .rktl, .rktd file extensions
- Add test_racket_search test
- Remove Kotlin from supported languages (was disabled)
- Clean up duplicate test files

Supported languages: Rust, Python, JavaScript, TypeScript, Go, Java, C, C++, Racket

crates/g3-core/Cargo.toml

@@ -40,6 +40,7 @@ tree-sitter-cpp = "0.23"
 # tree-sitter-kotlin = "0.3"  # Temporarily disabled - incompatible with tree-sitter 0.24
 tree-sitter-haskell = { git = "https://github.com/tree-sitter/tree-sitter-haskell" }
 tree-sitter-scheme = "0.24"
+tree-sitter-racket = "0.24"
 streaming-iterator = "0.1"
 walkdir = "2.4"