Pin pnpm via packageManager instead of per-workflow version

pnpm/action-setup@v4 now hard-requires an explicit version (no more
auto-detection fallback), which broke every workflow here except
tests.yml's hardcoded `version: 9` — discovered when the new release.yml's
workflow_dispatch dry run failed on "No pnpm version is specified."
Adding packageManager to package.json is the single source of truth
pnpm/action-setup reads automatically, so drop the now-redundant explicit
version from tests.yml too rather than maintaining it in two places.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: shixish

.github/workflows/tests.yml

@@ -15,8 +15,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: pnpm/action-setup@v4
-        with:
-          version: 9
       - uses: actions/setup-node@v4
         with:
           node-version: ${{ matrix.node-version }}
@@ -32,8 +30,6 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: pnpm/action-setup@v4
-        with:
-          version: 9
       - uses: actions/setup-node@v4
         with:
           node-version: 22

package.json

@@ -3,6 +3,7 @@
   "version": "0.2.0",
   "description": "TypeScript client library for Diffbot APIs",
   "type": "module",
+  "packageManager": "pnpm@10.33.0",
   "main": "./dist/index.cjs",
   "module": "./dist/index.js",
   "types": "./dist/index.d.ts",