fix(agent): surface tool calls before input completes

Emit tool-call placeholders as soon as tool input streaming starts so long writes appear immediately in chat. Reuse the same UI tool message when full input arrives to avoid duplicate cards, and keep the hook-required test suite green.

internal/models/http_client_test.go

@@ -10,6 +10,7 @@ func TestNewProviderHTTPClientWithoutTimeoutKeepsStreamingFriendlyBehavior(t *te
 	client := NewProviderHTTPClient(0)
 	if client == nil {
 		t.Fatal("expected client")
+		return
 	}
 	if client.Timeout != 0 {
 		t.Fatalf("expected no client timeout, got %s", client.Timeout)
@@ -29,6 +30,7 @@ func TestNewProviderHTTPClientWithTimeout(t *testing.T) {
 	client := NewProviderHTTPClient(timeout)
 	if client == nil {
 		t.Fatal("expected client")
+		return
 	}
 	if client.Timeout != timeout {
 		t.Fatalf("expected timeout %s, got %s", timeout, client.Timeout)