fix(ci): activate virtual environment for pytest and prek

Add venv activation step that adds .venv/bin (or Scripts on Windows) to
GITHUB_PATH. This ensures non-uv tools like pytest and prek use the
correct Python interpreter and installed packages from the virtual
environment.

Without this, pytest and prek would use the system Python and not find
the test dependencies installed by uv pip.

Author: claude

.github/workflows/tests.yml

@@ -32,6 +32,14 @@ jobs:
           python-version: ${{ matrix.python-version }}
       - name: Create virtual environment
         run: uv venv
+      - name: Activate virtual environment
+        run: |
+          if [ "$RUNNER_OS" == "Windows" ]; then
+            echo "$GITHUB_WORKSPACE/.venv/Scripts" >> $GITHUB_PATH
+          else
+            echo "$GITHUB_WORKSPACE/.venv/bin" >> $GITHUB_PATH
+          fi
+        shell: bash
       - name: Install Package
         run: uv pip install ".[test]"
       - name: Run pytest
@@ -51,6 +59,8 @@ jobs:
           install-only: true
       - name: Create virtual environment
         run: uv venv
+      - name: Activate virtual environment
+        run: echo "$GITHUB_WORKSPACE/.venv/bin" >> $GITHUB_PATH
       - name: Install Package
         run: uv pip install ".[test]"
       - name: run prek with plugin