azd tool: extract package manager command builders into extensible registry

## Summary

The `buildManagerCommand` function in `installer.go` uses a hardcoded switch statement for package managers (winget, brew, apt, npm, code). Adding new managers (choco, scoop, dnf, pacman, snap) requires modifying this switch. Notably, `snap` is already listed in `platformManagers` but has no corresponding case in `buildManagerCommand`, causing a silent failure.

## Proposal

Define a `PackageManagerStrategy` interface or a `map[string]CommandBuilder` registry that new managers can be added to without modifying the switch.

## Acceptance Criteria

- [ ] New package managers can be added without modifying `buildManagerCommand`
- [ ] `snap` support is either properly implemented or explicitly documented as unsupported
- [ ] Existing package manager behavior is unchanged

## Related

- Epic: #7845
- PR: #7450