[management,client] Default to NetBird's upload service when nothing is configured

The previous commit made a peer with no destination — no MDM override, no URL
named by the caller, nothing published by its management server — refuse to
upload and keep the bundle local unless it was enrolled with NetBird's cloud.
That closed the reported data-boundary concern, but it broke the default for
everyone who uploads a bundle as part of their day: a self-hosted user opening
a support ticket got a refusal where the command used to work.

Product decision (NetBird's, not the reporter's): the knob to keep bundles
inside your own infrastructure is what this branch provides, and it is enough.
The default stays the service NetBird runs, self-hosted included. An admin who
needs the bundles to stay in-house configures the destination; until then the
everyday flow keeps working.

So ResolveUploadURL drops the cloud check, the sentinel error and the
managementURL argument, and never fails:

    MDM  >  explicitly named URL  >  published by management  >  NetBird's service

Nothing observable changes for a deployment that configures nothing, which also
removes two edge cases the fail-closed default had: a peer still enrolled on the
legacy api.wiretrustee.com host would have been classified self-hosted and
refused, and an upgrade would have silently stopped uploads for self-hosted
deployments relying on them. The privilege gate is unaffected — a host other
than the default one still requires a privileged caller, so pointing the CLI
somewhere other than what management published needs root.

shared/management/http/api/openapi.yml

@@ -386,9 +386,9 @@ components:
         debug_bundle_upload_url:
           description: |
             Upload service the peers of this account send debug bundles to. A bundle carries peer logs, routes, DNS and
-            firewall state, so setting this keeps that data inside infrastructure the account controls instead of the
-            upload service NetBird runs. Must be an https URL with a host. Empty falls back to the deployment-wide value
-            configured on the management server; with neither, only peers enrolled with NetBird's cloud upload at all.
+            firewall state, so setting this keeps that data inside infrastructure the account controls. Must be an https
+            URL with a host. Empty falls back to the deployment-wide value configured on the management server, and with
+            neither to the upload service NetBird runs.
           type: string
           example: "https://upload.example.com/upload-url"
         agent_network_only:

shared/management/http/api/types.gen.go

@@ -1687,9 +1687,9 @@ type AccountSettings struct {
 	DashboardFeatures *AccountDashboardFeatures `json:"dashboard_features,omitempty"`
 
 	// DebugBundleUploadUrl Upload service the peers of this account send debug bundles to. A bundle carries peer logs, routes, DNS and
-	// firewall state, so setting this keeps that data inside infrastructure the account controls instead of the
-	// upload service NetBird runs. Must be an https URL with a host. Empty falls back to the deployment-wide value
-	// configured on the management server; with neither, only peers enrolled with NetBird's cloud upload at all.
+	// firewall state, so setting this keeps that data inside infrastructure the account controls. Must be an https
+	// URL with a host. Empty falls back to the deployment-wide value configured on the management server, and with
+	// neither to the upload service NetBird runs.
 	DebugBundleUploadUrl *string `json:"debug_bundle_upload_url,omitempty"`
 
 	// DnsDomain Allows to define a custom dns domain for the account