fix(mutation-tests): add inPlace:true to fix sandbox out/ directory issue

Stryker by default copies only git-tracked files to its sandbox.
Since \out/\ is gitignored, the compiled test files and vscode-shim
were missing from the sandbox, causing every test command to hang
until Stryker's per-mutant timeout was reached.

Setting inPlace:true makes Stryker mutate the compiled JS files
directly in the working directory (no sandbox), which works correctly
since npm run compile-tests already runs before npx stryker run.
Stryker still backs up the originals in .stryker-tmp before each
mutation and restores them afterwards.

Verified locally: html.js run completed in 5s with 0 timeouts.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: rajbos

vscode-extension/stryker.config.mjs

@@ -21,6 +21,16 @@ export default {
     ].join(' '),
   },
 
+  // inPlace: true — Stryker mutates the compiled JS files directly in the
+  // working directory instead of copying them to a sandbox. This is required
+  // because the `out/` directory is gitignored and would be excluded from the
+  // default sandbox, causing every test command to fail with MODULE_NOT_FOUND
+  // and produce misleading "timed out" results.
+  //
+  // Tradeoff: if Stryker crashes mid-run, files in out/src/ are left in a
+  // mutated state. Run `npm run compile-tests` to restore them.
+  inPlace: true,
+
   // Mutate compiled JS produced by `npm run compile-tests`.
   // The compiled tests in out/test/unit/ import from out/src/ via relative paths,
   // so mutating out/src/ is picked up by the test runner automatically.