fix: surface compose parser errors during prepare

[onyedikachi-david]

Summary

Surface Docker Compose parser errors instead of returning zero services.

Motivation

OpenShip Desktop could stay loading when Compose parsing failed, such as for a missing ${VAR:?message} value. The API swallowed the error, so the UI received no actionable failure.

Related issue

None.

Changes

apps/api

• Return a contextual Compose parser error.
• Add tests for missing required variables and invalid YAML.

Verification

bun run --cwd apps/api test -- test/modules/deployments/prepare.service.test.ts
# 3 tests passed

bun run --cwd apps/api lint
# passed

git diff --check
# passed

Checklist

• One change per PR
• The diff is scoped
• A regression test covers the bug
• Full monorepo tests and formatting pass
• I understand the diff