cmd: add retry/backoff (#10069)

This commit adds retry/backoff to the registry client for pull requests.

Also, revert progress indication to match original client's until we can
"get it right."

Also, make WithTrace wrap existing traces instead of clobbering them.
This allows clients to compose traces.

server/internal/client/ollama/trace.go

@@ -34,10 +34,27 @@ func (t *Trace) update(l *Layer, n int64, err error) {
 
 type traceKey struct{}
 
-// WithTrace returns a context derived from ctx that uses t to report trace
-// events.
+// WithTrace adds a trace to the context for transfer progress reporting.
 func WithTrace(ctx context.Context, t *Trace) context.Context {
-	return context.WithValue(ctx, traceKey{}, t)
+	old := traceFromContext(ctx)
+	if old == t {
+		// No change, return the original context. This also prevents
+		// infinite recursion below, if the caller passes the same
+		// Trace.
+		return ctx
+	}
+
+	// Create a new Trace that wraps the old one, if any. If we used the
+	// same pointer t, we end up with a recursive structure.
+	composed := &Trace{
+		Update: func(l *Layer, n int64, err error) {
+			if old != nil {
+				old.update(l, n, err)
+			}
+			t.update(l, n, err)
+		},
+	}
+	return context.WithValue(ctx, traceKey{}, composed)
 }
 
 var emptyTrace = &Trace{}