fix: don't mutate params in kb connectors (breaks retries)

Addresses PR review (Codex P2). The engine resolves a step's params once
and reuses that map across retry attempts, so `delete(params,
"_credential")` in kb/upsert and kb/query made attempt 2+ fail with a
missing credential instead of retrying a transient DB error. Read the
credential without mutating the shared map. Adds regression tests
asserting _credential survives a failed Execute.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WnKgCRfMvFcZAkoVbr8k79

Author: claude

packages/engine/internal/connector/kb.go

@@ -287,7 +287,6 @@ func (c *KBUpsertConnector) Execute(ctx context.Context, params map[string]any)
 	if err != nil {
 		return nil, fmt.Errorf("kb/upsert: %w", err)
 	}
-	delete(params, "_credential")
 
 	query, args, err := prepareUpsert(params)
 	if err != nil {
@@ -316,7 +315,6 @@ func (c *KBQueryConnector) Execute(ctx context.Context, params map[string]any) (
 	if err != nil {
 		return nil, fmt.Errorf("kb/query: %w", err)
 	}
-	delete(params, "_credential")
 
 	query, args, err := prepareQuery(params)
 	if err != nil {

packages/engine/internal/connector/kb_test.go

@@ -1,10 +1,47 @@
 package connector
 
 import (
+	"context"
 	"strings"
 	"testing"
 )
 
+// A malformed connection string so pgx.Connect fails immediately at parse time
+// (no network), letting us assert the connector didn't mutate its params.
+const kbBadCredURL = "postgres://bad host"
+
+// The engine resolves a step's params once and reuses that map across retry
+// attempts, so a connector must not mutate it — deleting _credential would make
+// attempt 2 fail with a missing credential instead of retrying.
+func TestKBUpsert_DoesNotMutateCredential(t *testing.T) {
+	params := map[string]any{
+		"_credential": map[string]string{"url": kbBadCredURL},
+		"table":       "kb_documents",
+		"content":     "x",
+		"vector":      "[1]",
+	}
+	if _, err := (&KBUpsertConnector{}).Execute(context.Background(), params); err == nil {
+		t.Fatal("expected a connection error")
+	}
+	if _, ok := params["_credential"]; !ok {
+		t.Error("_credential was removed from params (would break retries)")
+	}
+}
+
+func TestKBQuery_DoesNotMutateCredential(t *testing.T) {
+	params := map[string]any{
+		"_credential": map[string]string{"url": kbBadCredURL},
+		"table":       "kb_documents",
+		"vector":      "[1]",
+	}
+	if _, err := (&KBQueryConnector{}).Execute(context.Background(), params); err == nil {
+		t.Fatal("expected a connection error")
+	}
+	if _, ok := params["_credential"]; !ok {
+		t.Error("_credential was removed from params (would break retries)")
+	}
+}
+
 func TestPrepareUpsert_SingleRow(t *testing.T) {
 	sql, args, err := prepareUpsert(map[string]any{
 		"table":   "kb_documents",