Fix vitest command execution in test scripts

Change test:unit scripts to use `pnpm vitest` instead of `vitest` to
ensure vitest is properly resolved from pnpm's dependency tree.

The `vitest` command alone was failing with "Command not found" error
when executed through dotenvx, even though vitest is in devDependencies.
Using `pnpm vitest` ensures the command runs through pnpm's shim system.

- Update test:unit to use `pnpm vitest run`
- Update test:unit:update to use `pnpm vitest run --update`
- Update test:unit:coverage to use `pnpm vitest run --coverage`

This allows test scripts to run successfully via `pnpm test` command.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jdalton

package.json

@@ -75,9 +75,9 @@
     "bs": "pnpm run build:dist:src; pnpm exec socket --",
     "s": "pnpm exec socket --",
     "test": "run-s check test:*",
-    "test:unit": "dotenvx -q run -f .env.test -- vitest run --config .config/vitest.config.mts",
-    "test:unit:update": "dotenvx -q run -f .env.test -- vitest run --config .config/vitest.config.mts --update",
-    "test:unit:coverage": "dotenvx -q run -f .env.test -- vitest run --config .config/vitest.config.mts --coverage",
+    "test:unit": "dotenvx -q run -f .env.test -- pnpm vitest run --config .config/vitest.config.mts",
+    "test:unit:update": "dotenvx -q run -f .env.test -- pnpm vitest run --config .config/vitest.config.mts --update",
+    "test:unit:coverage": "dotenvx -q run -f .env.test -- pnpm vitest run --config .config/vitest.config.mts --coverage",
     "test:validate": "node scripts/validate-tests.mjs",
     "test:wrapper": "node scripts/test-wrapper.mjs",
     "test-ci": "run-s test:*",