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/state/migrations/0019_thread_dynamic_tools_defer_loading.sql

@@ -0,0 +1,2 @@
+ALTER TABLE thread_dynamic_tools
+ADD COLUMN defer_loading INTEGER NOT NULL DEFAULT 0;