bug(driver-mxc): test helper fails to compile after gateway-name argument #4191

User Story

I use an OpenShell source checkout on Windows to run the project's MSVC validation. Running windows:lint:x64 directly encountered a compile error in the MXC driver's test helper, so I cannot complete Windows lint or reach the test step.

Problem Statement

MxcComputeBackend::new now requires (gateway_name, config), but the test-only MxcComputeBackend::new_mocked helper still calls Self::new(config) in crates/openshell-driver-mxc/src/driver.rs:458. The Windows lint task includes all targets, so it compiles this #[cfg(test)] call and fails with E0277 and E0061.

Impact / Why This Matters

The current OpenShell source cannot complete Windows MSVC lint on x64; the same test target is part of the ARM64 lint task. Validation stops before Windows tests can run. Developers must patch the source locally to continue, which means the checked-in validation lane cannot confirm unrelated changes.

Acceptance Criteria

• The MXC library test target compiles during Windows MSVC lint on x64 and ARM64 without E0277 or E0061.
• The mocked backend uses a gateway name consistent with the driver's other test constructors.
• The Windows MSVC test steps can run after lint passes.

Reproduction Steps

1. Check out OpenShell main at 71c3cd957abef062eb7f37010056717cd49f2ed3 on a Windows machine with the repository's MSVC prerequisites.
2. Run mise run --skip-tools windows:lint:x64. On a native ARM64 machine, use windows:lint:arm64.
3. Observe the compile errors at crates/openshell-driver-mxc/src/driver.rs:458.

Environment

• OpenShell: main at 71c3cd957abef062eb7f37010056717cd49f2ed3
• OS: Windows, x64 MSVC validation
• Rust: 1.95.0 from rust-toolchain.toml
• Deployment: none required; this is a source checkout lint failure

Logs

error[E0277]: the trait bound `String: From<MxcComputeConfig>` is not satisfied
  --> crates/openshell-driver-mxc/src/driver.rs:458:37
error[E0061]: this function takes 2 arguments but 1 argument was supplied
  --> crates/openshell-driver-mxc/src/driver.rs:458:27
458 | let mut backend = Self::new(config);

Investigation

Commit a875add23 in PR #3264 added the gateway-name parameter to MxcComputeBackend::new for ETW event attribution but did not update new_mocked. The MXC implementation is gated to Windows, so non-Windows checks do not compile this call. Other MXC tests already use openshell_core::config::DEFAULT_GATEWAY_NAME when constructing the backend.