[management] Put the debug upload URL's https-and-host rule in the OpenAPI schema

The account handler rejects a non-empty value unless url.Parse yields scheme
https and a host, but the schema said only "string", so a schema-driven consumer
saw an unconstrained field and learned the contract from a 422. Add a pattern
that allows "" for the fallback and otherwise requires https with a host.

Regenerating types.gen.go produces no diff: our oapi-codegen config emits types
only, so the pattern documents the contract rather than enforcing it in Go. The
handler stays the thing that rejects.

Reported by CodeRabbit and cubic on #7514.

shared/management/http/api/openapi.yml

@@ -390,6 +390,7 @@ components:
             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
+          pattern: '^$|^https://[^/?#:\s]+(:[0-9]+)?([/?#].*)?$'
           example: "https://upload.example.com/upload-url"
         agent_network_only:
           description: Limits the dashboard to the Agent Network surface for this account. Set for accounts created via netbird.ai signups and can be disabled later. Enabling this requires dashboard_features.agent_network to be true in the same request.