Refactor devnet tests: move API keys to environment variables

- Add dotenvy dependency for .env file support
- Create .gitignore to exclude .env files from version control
- Update devnet-client-create-cmint.rs to load HELIUS_API_KEY from env
- Add debug output to client-close-ctoken.rs
- Fix function names in client-create-cmint.rs (derive_cmint_compressed_address)

Author: tilo-14

c-token/.gitignore

@@ -0,0 +1,5 @@
+# Ignore local environment and secrets
+.env
+.env*
+# Ignore build artifacts
+/target/