Move Rust testing documentation to main README

All Evergreen CI testing documentation for the Rust extension has been
consolidated into the main README.md in the Rust Extension section.

- Removed .evergreen/RUST_TESTING.md
- Added comprehensive Evergreen CI testing section to README.md
- Includes test configuration, environment variables, execution flow
- Includes troubleshooting guide and instructions for adding new tests

Author: aclark4life

.evergreen/RUST_TESTING.md

@@ -627,4 +627,90 @@ For implementation details, see the source code at `bson/_rbson/src/lib.rs`. Key
 
 **Investigation Complete:** The Rust extension successfully demonstrates that Rust is a viable alternative to C for Python extensions, with the trade-off being development complexity vs. performance. The investigation has identified the exact architectural differences that cause the performance gap and the effort required to close it.
 
+### Evergreen CI Testing
+
+The Rust BSON extension is automatically tested in Evergreen CI to ensure 100% compatibility with the C extension across all supported platforms and Python versions.
+
+#### Test Configuration
+
+**Automated Tests run on:**
+
+1. **Linux (RHEL 8)** - Primary platform, runs on every PR
+   - Python 3.10 on standalone (noauth-nossl) - **PR task**
+   - Python 3.12 on replica_set (noauth-ssl)
+   - Python 3.14 on sharded_cluster (auth-ssl)
+
+2. **macOS ARM64** - Weekly batch
+   - Same Python versions and topologies as Linux
+   - Important for M1/M2 Mac compatibility
+
+3. **Windows** - Weekly batch
+   - Same Python versions and topologies as Linux
+   - Important for cross-platform compatibility
+
+#### Environment Variables
+
+The following environment variables control Rust extension building and testing:
+
+- `PYMONGO_BUILD_RUST=1` - Build the Rust extension during installation
+- `PYMONGO_USE_RUST=1` - Use the Rust extension for BSON operations
+
+These are automatically set by the Evergreen configuration for Rust test tasks.
+
+#### Test Execution Flow
+
+1. **Setup Phase** (`.evergreen/scripts/setup-tests.sh`)
+   - Detects `PYMONGO_BUILD_RUST` environment variable
+   - Installs Rust toolchain via `.evergreen/scripts/install-rust.sh`
+   - Installs maturin (Rust-Python build tool)
+
+2. **Build Phase**
+   - PyMongo is installed with `PYMONGO_BUILD_RUST=1`
+   - This triggers the Rust extension build via `build.py`
+   - The Rust extension is compiled and installed alongside the C extension
+
+3. **Test Phase** (`.evergreen/scripts/run-tests.sh`)
+   - Tests run with `PYMONGO_USE_RUST=1`
+   - This causes PyMongo to use the Rust extension instead of C
+   - Full BSON test suite runs to ensure 100% compatibility
+
+#### Test Coverage
+
+The Rust extension tests run the full BSON test suite (`test/test_bson.py`) which includes:
+
+- All BSON types (int, string, document, array, binary, ObjectId, datetime, etc.)
+- Edge cases and error handling
+- Encoding/decoding round-trips
+- Codec options and type preservation
+- Performance characteristics
+
+#### Adding New Rust Tests
+
+To add new test configurations:
+
+1. Edit `.evergreen/scripts/generate_config.py`
+2. Modify `create_rust_extension_tasks()` to add new Python versions or topologies
+3. Modify `create_rust_extension_variants()` to add new platforms
+4. Run `python .evergreen/scripts/generate_config.py` to regenerate configs
+5. Commit the changes to `.evergreen/generated_configs/`
+
+#### Troubleshooting CI Tests
+
+**Rust Not Found:**
+- Check that `.evergreen/scripts/install-rust.sh` is executable
+- Verify the script runs successfully in the setup phase
+- Check Evergreen logs for Rust installation errors
+
+**Build Failures:**
+- Check that maturin is installed (`maturin --version`)
+- Verify Rust toolchain is up to date (`rustc --version`)
+- Check for compilation errors in the build logs
+- Ensure PyO3 dependencies are compatible
+
+**Test Failures:**
+- Verify `PYMONGO_USE_RUST=1` is set
+- Check that the Rust extension is actually loaded (run `just rust-check`)
+- Compare behavior with C extension (run tests without `PYMONGO_USE_RUST`)
+- Check for platform-specific issues (endianness, pointer size, etc.)
+
 ---