fix: avoid mutating proto App.Healthcheck in insertAgentApp (#22954)

## Problem

`insertAgentApp` mutated its input by writing to `app.Healthcheck` when
it was nil (line 3525):

```go
if app.Healthcheck == nil {
    app.Healthcheck = &sdkproto.Healthcheck{}  // mutation!
}
```

The Devcontainers subtests share the same `tt.resource` pointer across
two parallel goroutines (`WithProtoIDs` and `WithoutProtoIDs`), causing
a data race on the `Healthcheck` field (and its sub-fields `Url`,
`Interval`, `Threshold`).

## Fix

Replace the in-place mutation with a local variable:

```go
healthcheck := app.GetHealthcheck()
if healthcheck == nil {
    healthcheck = &sdkproto.Healthcheck{}
}
```

This avoids writing back to the shared proto message. All downstream
reads now use the local `healthcheck` variable.

coderd/provisionerdserver/provisionerdserver.go

@@ -3521,10 +3521,11 @@ func insertAgentApp(ctx context.Context, db database.Store, agentID uuid.UUID, a
 	appSlugs[slug] = struct{}{}
 
 	health := database.WorkspaceAppHealthDisabled
-	if app.Healthcheck == nil {
-		app.Healthcheck = &sdkproto.Healthcheck{}
+	healthcheck := app.GetHealthcheck()
+	if healthcheck == nil {
+		healthcheck = &sdkproto.Healthcheck{}
 	}
-	if app.Healthcheck.Url != "" {
+	if healthcheck.Url != "" {
 		health = database.WorkspaceAppHealthInitializing
 	}
 
@@ -3579,9 +3580,9 @@ func insertAgentApp(ctx context.Context, db database.Store, agentID uuid.UUID, a
 		External:             app.External,
 		Subdomain:            app.Subdomain,
 		SharingLevel:         sharingLevel,
-		HealthcheckUrl:       app.Healthcheck.Url,
-		HealthcheckInterval:  app.Healthcheck.Interval,
-		HealthcheckThreshold: app.Healthcheck.Threshold,
+		HealthcheckUrl:       healthcheck.Url,
+		HealthcheckInterval:  healthcheck.Interval,
+		HealthcheckThreshold: healthcheck.Threshold,
 		Health:               health,
 		// #nosec G115 - Order represents a display order value that's always small and fits in int32
 		DisplayOrder: int32(app.Order),