GitHub Action: azure/setup-cosmosdb-shell for CI workflows

## Feedback source
External user feedback (travel multi-agent workshop team).

## Problem
There is no official GitHub Action to install/setup CosmosDB Shell in CI workflows. Users must manually download, extract, and add to PATH — adding boilerplate and fragility to every workflow.

## Proposed solution
Publish an official **azure/setup-cosmosdb-shell** GitHub Action that:
- Accepts a version input (with a sensible default/latest)
- Downloads the correct platform-specific package
- Extracts and adds to PATH
- Caches the binary across runs (using actions/cache or tool-cache)
- Verifies checksum/signature

## Example usage
```yaml
steps:
- uses: azure/setup-cosmosdb-shell@v1
with:
version: 1.0.213-preview
```n
## Acceptance criteria
- [ ] Action published to GitHub Marketplace
- [ ] Supports Linux, macOS, Windows runners
- [ ] Built-in caching (tool-cache pattern)
- [ ] Version pinning and 'latest' support
- [ ] Documented in CI/CD guide

Commencez par le workflow d’exemple dans l’issue et consultez le guide CI/CD pour comprendre la configuration attendue côté utilisateur. Le travail est terminé lorsqu’une action azure/setup-cosmosdb-shell publiée prend en charge les plateformes et les modes de version listés, la mise en cache, la vérification et l’utilisation documentée de CI/CD.

Rédigé par le modèle d'indexation à partir du texte de l'issue.