Add FileDescriptorId abstraction for serializable file descriptors

[dlunch]

Summary

• Introduce FileDescriptorId newtype wrapping usize to replace raw Rust object pointer storage in FileDescriptor, making it serializable
• Update Runtime trait so stdin/stdout/stderr/open return FileDescriptorId instead of Box<dyn File>, with a new get_file(fd) method for retrieval
• Add fd table (Arc<Mutex<BTreeMap>>) to RuntimeImpl and TestRuntime for fd → file mapping management

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[Copilot]

Pull request overview

This PR replaces storing raw Rust object pointers in java.io.FileDescriptor with a FileDescriptorId newtype, enabling file descriptor values to be represented as plain integers (and thus more amenable to serialization) while allowing runtimes to map IDs back to File objects.

Changes:

• Introduces FileDescriptorId(usize) and re-exports it from java_runtime.
• Updates the Runtime trait: stdin/stdout/stderr/open now return FileDescriptorId, plus a new get_file(fd) method for lookup.
• Adds fd tables (Arc<Mutex<BTreeMap<..>>>) to RuntimeImpl and TestRuntime, and updates Java I/O classes to use the new fd-based flow.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
java_runtime/src/runtime/io.rs	Adds the FileDescriptorId newtype used as the runtime-facing FD handle.
java_runtime/src/runtime.rs	Updates the Runtime trait to return FileDescriptorId and adds get_file.
java_runtime/src/lib.rs	Re-exports FileDescriptorId from the java_runtime crate API.
src/runtime.rs	Implements an fd table in RuntimeImpl and adapts stdio/open to return ids and support lookup.
test_utils/src/lib.rs	Updates TestRuntime to use an fd table and the new Runtime interface.
java_runtime/src/classes/java/io/file_descriptor.rs	Changes Java FileDescriptor storage from raw pointer bytes to an int fd and resolves via Runtime::get_file.
java_runtime/src/classes/java/io/random_access_file.rs	Switches to creating FileDescriptor from a FileDescriptorId and passes runtime context for lookup.
java_runtime/src/classes/java/io/file_input_stream.rs	Switches open/init and read paths to fd-based lookup via Runtime::get_file.
java_runtime/src/classes/java/io/file_output_stream.rs	Switches open/init and write paths to fd-based lookup via Runtime::get_file.

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