Refactor Rust extension into modular structure

Split lib.rs (3,117 lines) into 6 well-organized modules:
- lib.rs: Module exports and public API (76 lines)
- types.rs: Type cache and BSON type markers (266 lines)
- errors.rs: Error handling utilities (56 lines)
- utils.rs: Utility functions (154 lines)
- encode.rs: BSON encoding functions (1,545 lines)
- decode.rs: BSON decoding functions (1,141 lines)

Benefits:
- Better code organization and maintainability
- Follows Rust best practices for module structure
- Improved compilation times (parallel module compilation)
- Easier code navigation and testing
- Clear separation of concerns

All tests passing: test/test_bson.py (86 passed, 2 skipped)
Tests for unimplemented features properly skipped via @skip_if_rust_bson

Author: aclark4life

bson/_rbson/src/decode.rs

@@ -0,0 +1,55 @@
+// Copyright 2025-present MongoDB, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Error handling utilities for BSON operations
+
+use pyo3::prelude::*;
+use pyo3::types::{PyAny, PyTuple};
+
+use crate::types::TYPE_CACHE;
+
+/// Helper to create InvalidDocument exception
+pub(crate) fn invalid_document_error(py: Python, msg: String) -> PyErr {
+    let invalid_document = TYPE_CACHE.get_invalid_document_class(py)
+        .expect("Failed to get InvalidDocument class");
+    PyErr::from_value(
+        invalid_document.bind(py)
+            .call1((msg,))
+            .expect("Failed to create InvalidDocument")
+    )
+}
+
+/// Helper to create InvalidDocument exception with document property
+pub(crate) fn invalid_document_error_with_doc(py: Python, msg: String, doc: &Bound<'_, PyAny>) -> PyErr {
+    let invalid_document = TYPE_CACHE.get_invalid_document_class(py)
+        .expect("Failed to get InvalidDocument class");
+    // Call with positional arguments: InvalidDocument(message, document)
+    let args = PyTuple::new_bound(py, &[msg.into_py(py), doc.clone().into_py(py)]);
+    PyErr::from_value(
+        invalid_document.bind(py)
+            .call1(args)
+            .expect("Failed to create InvalidDocument")
+    )
+}
+
+/// Helper to create InvalidBSON exception
+pub(crate) fn invalid_bson_error(py: Python, msg: String) -> PyErr {
+    let invalid_bson = TYPE_CACHE.get_invalid_bson_class(py)
+        .expect("Failed to get InvalidBSON class");
+    PyErr::from_value(
+        invalid_bson.bind(py)
+            .call1((msg,))
+            .expect("Failed to create InvalidBSON")
+    )
+}