fix(ci): install client from release branch before uv sync (#4002)

Backport of #4001 to release-0.3.x branch.

Fixes CI failures on release branches where uv sync can't resolve RC
dependencies.

## The Problem

On release branches like `release-0.3.x`, pyproject.toml requires
`llama-stack-client>=0.3.1rc1`. RC versions only exist on test.pypi, not
PyPI. This causes multiple CI failures:

1. `uv sync` fails because it can't resolve RC versions from PyPI
2. pre-commit hooks (uv-lock, codegen) fail for the same reason  
3. mypy workflow section needs uv installed

## The Solution

Configure UV to use test.pypi when on release branches:

- Set `UV_INDEX_URL=https://test.pypi.org/simple/` (primary)
- Set `UV_EXTRA_INDEX_URL=https://pypi.org/simple/` (fallback)
- Set `UV_INDEX_STRATEGY=unsafe-best-match` to check both indexes

This allows `uv sync` to resolve common packages from PyPI and RC
versions from test.pypi.

## Additional Fixes

- Export UV env vars to `GITHUB_ENV` so pre-commit hooks inherit them
- Install uv in pre-commit workflow for mypy section
- Handle missing `type_checking` dependency group on release-0.3.x
- Regenerate uv.lock with RC versions for the release branch

## Changes

- Created reusable `install-llama-stack-client` action for configuration
- Modified `setup-runner` to set UV environment variables before sync
- Modified `pre-commit` workflow to configure client and export env vars
- Updated uv.lock with RC versions from test.pypi

This is a cherry-pick of commits afa9f0882, c86e6e906, 626639bee, and
081566321 from main, plus additional fixes for release branch
compatibility.

scripts/integration-tests.sh

@@ -271,6 +271,16 @@ if [[ "$STACK_CONFIG" == *"docker:"* && "$COLLECT_ONLY" == false ]]; then
         --build-arg "LLAMA_STACK_DIR=/workspace"
     )
 
+    # Pass UV index configuration for release branches
+    if [[ -n "${UV_EXTRA_INDEX_URL:-}" ]]; then
+        echo "Adding UV_EXTRA_INDEX_URL to docker build: $UV_EXTRA_INDEX_URL"
+        build_cmd+=(--build-arg "UV_EXTRA_INDEX_URL=$UV_EXTRA_INDEX_URL")
+    fi
+    if [[ -n "${UV_INDEX_STRATEGY:-}" ]]; then
+        echo "Adding UV_INDEX_STRATEGY to docker build: $UV_INDEX_STRATEGY"
+        build_cmd+=(--build-arg "UV_INDEX_STRATEGY=$UV_INDEX_STRATEGY")
+    fi
+
     if ! "${build_cmd[@]}"; then
         echo "❌ Failed to build Docker image"
         exit 1