feat: explicit parallel deploy opt-in for non-Aspire templates

[jongio]

Problem

Currently, when no service declares uses: [other_service] in azure.yaml, all deploys run sequentially for backward compatibility. This means templates with fully independent services (no inter-service dependencies) cannot opt into parallel deployment without artificially declaring a uses: edge.

Aspire projects are now fixed (PR #7776) — the build-gate policy implicitly enables parallel mode. But non-Aspire templates with independent services have no way to opt in.

Proposed Solution

Add an explicit opt-in field in azure.yaml:

# Enables parallel deploy for all services (no sequential fallback)
execution:
  parallel: true

Or at the service-graph level:

services:
  api:
    ...
  web:
    ...
    uses: [api]  # explicit dep, web waits for api
  worker:
    ...
    # no uses: -> runs in parallel with api

Key Considerations

1. uses: [] cannot work — due to omitempty, an empty list is indistinguishable from an absent field at the YAML/Go level.
2. Backward compatibility — sequential must remain the default for existing templates that may rely on implicit ordering.
3. Scope — this affects deploy ordering. Provision parallelism is already controlled by infra.layers + dependsOn.
4. Extension support — the field should be generic enough for extensions to consume.

Context

• Raised by @vhvb1989 during PR feat(exegraph): graph-driven execution engine for up/provision/deploy #7776 review
• Aspire parallel was fixed separately via the build-gate policy mechanism
• The sequential fallback exists because pre-exegraph azd always deployed services sequentially, and some templates may depend on that ordering via hook env vars

Related

• PR feat(exegraph): graph-driven execution engine for up/provision/deploy #7776 (exegraph execution engine)
• cli/azd/internal/cmd/service_graph.go — hasExplicitOrdering logic