ci(python-setup): drop -U upgrade flag from uv sync (#5961)

The shared composite action ran `uv sync --all-packages --all-extras
--dev -U` on every job, which upgrades every dependency to the latest
compatible version instead of using the pinned versions in `uv.lock`.

That is currently producing a hard resolver failure on every CI job:

    No solution found when resolving dependencies for split
    (markers: python_full_version >= '3.11' and sys_platform == 'darwin')
    Because there are no versions of durabletask and
    agent-framework-durabletask depends on durabletask>=1.3.0,<2,
    we can conclude that agent-framework-durabletask's requirements
    are unsatisfiable.

Dropping `-U` makes the install use the workspace lockfile, which is
what is reproducible locally and what we publish releases against.
Upgrades should be opt-in (via a scheduled job or a separate workflow)
rather than implicit on every CI run.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/actions/python-setup/action.yml

@@ -40,4 +40,4 @@ runs:
       - name: Install the project
         shell: bash
         run: |
-          cd python && uv sync --all-packages --all-extras --dev -U --prerelease=if-necessary-or-explicit
+          cd python && uv sync --all-packages --all-extras --dev --prerelease=if-necessary-or-explicit