Restore the cargo test check for branch protection

The main branch ruleset requires a status check named exactly
`cargo test`. This PR had added a Windows build matrix to the
test-rust job, which renamed its check context to
`cargo test (ubuntu-latest)` and `cargo test (windows-latest)` and
left the required `cargo test` context unreported, so the PR stayed
blocked with "Expected, waiting for status to be reported".

Revert test-rust to its Linux-only form so it emits the `cargo test`
context again. Windows coverage remains through the axum-native,
cloudflare, and CLI jobs.

Author: jwrosewell

.github/workflows/test.yml

@@ -11,12 +11,8 @@ on:
 
 jobs:
   test-rust:
-    name: cargo test (${{ matrix.os }})
-    runs-on: ${{ matrix.os }}
-    strategy:
-      fail-fast: false
-      matrix:
-        os: [ubuntu-latest, windows-latest]
+    name: cargo test
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4