Fix CI highest resolution test to actually test highest versions

The "highest" dependency resolution test was using --frozen flag,
which prevented uv from updating dependencies to their highest
available versions. This meant the test was using whatever was
locked in uv.lock from the main branch, not the actual highest
versions.

This caused the CI to miss breaking changes in new dependency
versions (like pydantic 2.12) that were compatible with version
constraints but not yet in the lockfile.

Changes:
- Use --upgrade --resolution highest for the highest resolution test
  (--upgrade is needed to ignore the lockfile and re-resolve)
- Use --resolution lowest-direct for the lowest resolution test
- Simplify test execution to use --frozen --no-sync since
  resolution already happened during uv sync
- Remove redundant UV_RESOLUTION env var (command-line flags suffice)

This ensures the highest test catches compatibility issues with
newer dependency versions before they reach main.

Author: maxisbey

.github/workflows/shared.yml

@@ -40,7 +40,7 @@ jobs:
           - name: lowest-direct
             install-flags: "--resolution lowest-direct"
           - name: highest
-            install-flags: "--frozen"
+            install-flags: "--upgrade --resolution highest"
         os: [ubuntu-latest, windows-latest]
 
     steps:
@@ -56,9 +56,7 @@ jobs:
         run: uv sync ${{ matrix.dep-resolution.install-flags }} --all-extras --python ${{ matrix.python-version }}
 
       - name: Run pytest
-        run: uv run ${{ matrix.dep-resolution.install-flags }} --no-sync pytest
-        env:
-          UV_RESOLUTION: ${{ matrix.dep-resolution.name == 'lowest-direct' && 'lowest-direct' || 'highest' }}
+        run: uv run --frozen --no-sync pytest
 
   readme-snippets:
     runs-on: ubuntu-latest