refactor: flatten jvm_bridge submodule into jvm-bridge crate root

Move macros, JVMClasses, and helper types from the jvm_bridge submodule
to the crate root, eliminating the redundant jvm_bridge::jvm_bridge path.

Author: andygrove

native/core/src/lib.rs

@@ -56,9 +56,13 @@ use mimalloc::MiMalloc;
 
 // Re-export from jvm-bridge crate for internal use
 pub use datafusion_comet_jvm_bridge::errors;
-pub use datafusion_comet_jvm_bridge::jvm_bridge;
 pub use datafusion_comet_jvm_bridge::JAVA_VM;
 
+/// Re-export jvm-bridge items under the `jvm_bridge` name for convenience.
+pub mod jvm_bridge {
+    pub use datafusion_comet_jvm_bridge::*;
+}
+
 use errors::{try_unwrap_or_throw, CometError, CometResult};
 
 #[macro_use]