`azd app run` doesn't auto-detect Azure Functions for services with `host: staticwebapp`

[jongio]

Problem

Static Web Apps with managed Azure Functions is a very common Azure pattern — the SWA resource bundles a frontend and a managed Functions API. In azure.yaml, both services must use host: staticwebapp because they deploy to the same SWA resource:

services:
  web:
    project: .
    host: staticwebapp
    language: js
  api:
    project: api
    host: staticwebapp
    language: js

However, azd app run uses the host property to determine how to run services locally. In detector.go, DetectServiceRuntime() only triggers Functions handling when service.Host == "function":

// Special handling for Azure Functions (all variants including Logic Apps)
if service.Host == "function" {
    return buildFunctionsRuntime(serviceName, service, projectDir, usedPorts, azureYamlDir)
}

This means that when host: staticwebapp, the API service misses all Functions auto-detection:

1. ❌ func start is not launched for the API service
2. ❌ detectFunctionsVariant() (in functions.go) is never called — no v3/v4 model detection
3. ❌ InjectFunctionsWorkerRuntime() (in environment.go) is skipped — no FUNCTIONS_WORKER_RUNTIME, no local.settings.json Values injection
4. ❌ Functions /admin/host/status health check is not used
5. ❌ Functions port manager is not engaged

The host property is serving dual duty — deployment target AND local dev behavior — and these conflict for the SWA + managed Functions pattern.

Reproduction

1. Create a Static Web App project with a managed Functions API (e.g., React frontend + Azure Functions in api/)
2. In azure.yaml, set both services to host: staticwebapp
3. Run azd app run
4. Observe: the API service doesn't start with func start, doesn't get environment injection, and doesn't get the correct health check

Current Workaround

Users must manually configure what auto-detection would normally provide:

services:
  api:
    project: api
    host: staticwebapp
    language: js
    command: func start --port 7336
    ports: ["7336"]
    environment:
      AzureWebJobsStorage: UseDevelopmentStorage=true
      FUNCTIONS_WORKER_RUNTIME: node

This defeats the purpose of the auto-detection that azd app run provides for host: function services.

Proposed Solutions

Option 1 (Preferred): Auto-detect Functions within staticwebapp services

In DetectServiceRuntime(), when host: staticwebapp and the project directory contains Functions markers (host.json + @azure/functions in deps, function.json files, Python/C#/.NET Functions markers), call buildFunctionsRuntime() for local dev purposes.

The existing detectFunctionsVariant() logic already handles variant detection — it just needs to also be called for staticwebapp host services.

Option 2: Add a localHost or devHost override

services:
  api:
    host: staticwebapp       # For deployment
    localHost: function      # For azd app run

This separates the deployment target from the local dev behavior explicitly.

Option 3: SWA CLI integration

Detect the SWA + Functions pattern and use swa start to orchestrate both services, since the SWA CLI already handles the reverse proxy and Functions runtime natively.

Impact

This affects anyone using the very common pattern of Static Web Apps with managed Functions — a core Azure serverless pattern. Without this fix, users lose the main benefit of azd app run (zero-config local dev) and have to manually configure command, ports, and environment variables.