fix(llamaswap): address Gadfly review findings

- Unload: reject model ids containing path separators (/?#) so a model name
  can't redirect the request to another endpoint; ":" (common in ids) stays
  verbatim.
- doJSON: take a model arg so image/management HTTP errors carry the target id
  (was always ""); add a base-URL guard so management methods fail clearly
  instead of building a bare-path request; cap the success-path JSON decode with
  io.LimitReader (64 MiB) and drain the body when out is nil for conn reuse.
- image: reject negative Request.N before sending.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

provider/llamaswap/llamaswap_test.go

@@ -127,6 +127,24 @@ func TestUnload(t *testing.T) {
 	if gotPath != "/api/models/unload" {
 		t.Errorf("unload-all path = %q", gotPath)
 	}
+
+	// A model id with a path separator is rejected before any request.
+	if err := p.Unload(context.Background(), "../admin"); err == nil {
+		t.Error("expected error for model id with path separator")
+	}
+}
+
+func TestManagementNoBaseURL(t *testing.T) {
+	p := New() // no base URL
+	if _, err := p.ListModels(context.Background()); err == nil {
+		t.Error("ListModels: expected error for missing base URL")
+	}
+	if _, err := p.Running(context.Background()); err == nil {
+		t.Error("Running: expected error for missing base URL")
+	}
+	if err := p.Unload(context.Background(), "m"); err == nil {
+		t.Error("Unload: expected error for missing base URL")
+	}
 }
 
 func TestRunningRaw(t *testing.T) {