docs: M5d-2 tool_summary_index content is String

The secret-summary given test built ToolResult and ToolSummary with
&str content; those fields are String.

Co-Authored-By: Grok 4.6 <noreply@x.ai>

docs/plans/M5d2/files/crates/memoryd/tests/tool_summary_index.rs

@@ -164,7 +164,7 @@ fn a_secret_summary_is_skipped_when_the_index_is_private() {
                 time: logs::at(4),
                 call: CallId(1),
                 tool_call_id: "c0".to_string(),
-                content: "secret page",
+                content: "secret page".to_string(),
                 class: DataClass::Secret,
                 untrusted: false,
                 truncated: false,
@@ -173,7 +173,7 @@ fn a_secret_summary_is_skipped_when_the_index_is_private() {
                 time: logs::at(5),
                 call: CallId(1),
                 tool_call_id: "c0".to_string(),
-                content: "SECRET_SUMMARY_PHRASE",
+                content: "SECRET_SUMMARY_PHRASE".to_string(),
                 class: DataClass::Secret,
                 untrusted: false,
             },