fix(slack): align vitest with the repo version so blob reports merge

The slack package pinned its own vitest ^2.1.9 while the rest of the repo uses
4.1.7, so its test blob reports could not be merged with the others (the
packages Merge Reports step requires a single vitest version). Drop the pin so
slack inherits the root vitest like every other package.

Author: ericallam

packages/slack/package.json

@@ -51,8 +51,7 @@
     "@trigger.dev/sdk": "workspace:4.5.0-rc.7",
     "rimraf": "6.0.1",
     "tshy": "^3.0.2",
-    "tsx": "4.17.0",
-    "vitest": "^2.1.9"
+    "tsx": "4.17.0"
   },
   "engines": {
     "node": ">=18.20.0"