fix: resolve CI failures in test workflow

Changes:
- Fix test job to use 'uv sync' instead of 'uv pip install -e .'
  (uv pip requires a venv, uv sync creates one automatically)
- Add uv.lock file for reproducible builds

This resolves the 'No virtual environment found' errors across all
test matrix combinations (3 OSes × 5 Python versions).

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: claude

.github/workflows/test.yml

@@ -84,7 +84,7 @@ jobs:
         run: uv python pin ${{ matrix.python-version }}
 
       - name: Install package
-        run: uv pip install -e .
+        run: uv sync
 
       - name: Test CLI can be invoked
         run: uv run promptfoo --version