azd tool: support multiple install strategies per platform with fallback chain

## Summary

Each platform gets exactly one `InstallStrategy` in the tool manifest (`map[string]InstallStrategy`). If the primary package manager is unavailable (e.g., winget not installed on older Windows), the only fallback is a `FallbackUrl` that sends users to a web page. There is no automatic fallback to alternative package managers or direct download.

## Proposal

Change to `map[string][]InstallStrategy` with priority ordering. The installer iterates strategies until one succeeds or all are exhausted. This is an additive schema change.

## Acceptance Criteria

- [ ] Multiple install strategies can be defined per platform with priority ordering
- [ ] Installer attempts strategies in order, falling back on failure
- [ ] Existing single-strategy definitions continue to work unchanged

## Related

- Epic: #7845
- PR: #7450