Preserve longer MCP source descriptions in tool search (#37066)

## Why

The 4 KiB aggregate limit can truncate source metadata before it reaches the
model through the tool search description.

## What changed

Raise the aggregate source-description limit to 512 KiB while retaining the
existing character-boundary-safe truncation and bounded output.

## Testing

Expand unit and RMCP round-trip coverage to verify that substantially longer
Unicode descriptions remain complete within the new limit.

GitOrigin-RevId: 002760cdd2e8741e531b45f75f8624ef84334d24

codex-rs/core/tests/suite/rmcp_client.rs

@@ -620,7 +620,7 @@ async fn stdio_server_round_trip() -> anyhow::Result<()> {
     .await;
 
     let expected_env_value = "propagated-env";
-    let expected_description = format!("{}🦀keep the complete MCP metadata", "é".repeat(499));
+    let expected_description = format!("{}🦀keep the complete MCP metadata", "é".repeat(11_000));
     let instructions = expected_description.clone();
     let rmcp_test_server_bin = remote_aware_stdio_server_bin()?;
 
@@ -713,7 +713,7 @@ async fn stdio_server_round_trip() -> anyhow::Result<()> {
         .and_then(|tool| tool.get("description").and_then(Value::as_str))
         .expect("the model should receive a tool search description");
     assert!(
-        search_description.len() < 5 * 1024,
+        search_description.len() < 513 * 1024,
         "the complete tool search description must remain bounded"
     );
     assert!(search_description.contains(&format!("- rmcp: {expected_description}")));