dynamic tool calls: add param exposeToContext to optionally hide tool (#14501)

This extends dynamic_tool_calls to allow us to hide a tool from the
model context but still use it as part of the general tool calling
runtime (for ex from js_repl/code_mode)

codex-rs/app-server-protocol/schema/json/v2/ThreadStartParams.json

@@ -61,6 +61,9 @@
     },
     "DynamicToolSpec": {
       "properties": {
+        "deferLoading": {
+          "type": "boolean"
+        },
         "description": {
           "type": "string"
         },