Investigate cross-compiling the Windows ARM64 mock wheel from the x64 agent

Follow-up to #392, which merged the Linux libc-flavour pairs and took the sandbox publish pipeline's concurrent build agents from 7 to 5.

Idea

BuildWindows_arm64 could potentially be folded into BuildWindows_x64 by cross-compiling with maturin build --target aarch64-pc-windows-msvc. That would take peak concurrent build agents from 5 to 4 and free one more RUST-ARM64-WUS3 agent.

It would also delete the two Node.js bootstrap steps that only the ARM64 Windows agent needs:

- pwsh: ./scripts/setup-nodejs-path.ps1
  displayName: 'Add Node.js to PATH for ADO tasks'
- pwsh: ./scripts/install-nodejs-to-agent.ps1
  displayName: 'Install Node.js to agent externals folder'

Why it looks plausible

• mssql-mock-tds pulls the openssl crate only on cfg(not(windows)), and native-tls uses SChannel on Windows. So there is no C library to cross-build — the usual blocker for this kind of change is absent.
• pyo3 is configured abi3-py39. abi3 cross-compilation does not need a target-architecture interpreter, only PYO3_CROSS_PYTHON_VERSION.

Why it was not done in #392

Two things cannot be settled by reading the repo:

1. Does the RUST-W22-SQL25 image have the MSVC ARM64 cross tools (VC.Tools.ARM64)? Without them the link step fails, and that only surfaces in a real pipeline run.
2. Does the cross-built wheel actually load on Windows ARM64? Building natively today gives us an accidental smoke test that cross-compiling removes. Losing it silently would be worse than the agent saving is worth.

Both are answerable, but they need runs of a pipeline that has publish side effects. Bundling that speculation into #392 would have put a clean two-agent win at risk of a red run.

Suggested approach

1. Confirm VC.Tools.ARM64 is present on the RUST-W22-SQL25 image (or get it added).
2. Prototype the cross-build on a branch and confirm the wheel's platform tag is win_arm64.
3. Verify the resulting wheel imports and runs on a real Windows ARM64 machine before deleting the native job.
4. Only then merge the jobs.

If step 1 or 3 does not pan out, close this — keeping the native ARM64 job is a perfectly reasonable outcome.

Start by checking whether the RUST-W22-SQL25 image has VC.Tools.ARM64, then prototype BuildWindows_arm64 cross-compilation from BuildWindows_x64 with maturin and aarch64-pc-windows-msvc. Confirm the wheel is tagged win_arm64 and imports and runs on a real Windows ARM64 machine before removing the native job and its Node.js setup steps.