test(api): keep the package's tests off the live mojang profile api

mojangProfileAPI defaults to https://api.mojang.com, and only the tests
that call stubMojangNames or setProfileAPI swap it out. A new test that
reaches a third-party login without doing so would query the real
service: its result then depends on network access and on whether
someone owns the name that day, and the shared premium cache can carry
that answer into later tests.

A TestMain now points the lookup at an address nothing listens on
before any test runs, so a forgotten stub always takes the same
fail-closed path. Tests that stub it restore this address, not the live
one, when they finish.

internal/api/main_test.go

@@ -0,0 +1,15 @@
+package api
+
+import (
+	"os"
+	"testing"
+)
+
+// TestMain points the premium-name lookup at an address nothing listens on before any test
+// runs. A test that forgets stubMojangNames then fails the same way everywhere (lookup
+// error, fail closed, rename) instead of asking the real api.mojang.com, whose answer
+// changes the day someone buys the name and which CI may not reach at all.
+func TestMain(m *testing.M) {
+	mojangProfileAPI = "http://127.0.0.1:1/"
+	os.Exit(m.Run())
+}