fix(serialization): restore tools message key

## About

Restore the original serialized message schema for assistant tool calls by
writing parsed tool metadata back under the tools key. This keeps
serialized contexts aligned with the original format and adds focused
coverage for tool-call round trips.

## Changes

- serialize parsed message tool calls under the tools key again
- update message serialization coverage to expect the original key
- add a context round-trip spec for assistant tool-call payloads

Author: 0x1eef

lib/llm/message.rb

@@ -38,7 +38,7 @@ def to_h
         content:,
         reasoning_content:,
         compaction: extra.compaction,
-        tool_calls: extra.tool_calls&.map { LLM::Object === _1 ? _1.to_h : _1 },
+        tools: extra.tool_calls&.map { LLM::Object === _1 ? _1.to_h : _1 },
         usage:,
         original_tool_calls: extra.original_tool_calls
       }.compact.then { preserve_nil_content(_1) }

spec/context_spec.rb

@@ -189,6 +189,56 @@
         expect(content.fetch(2).value.file?).to eq(true)
         expect(content.fetch(2).value.id).to eq("file_123")
       end
+
+      context "with assistant tool calls" do
+        let(:message) do
+          LLM::Message.new("assistant", nil, {
+            tool_calls: [
+              {id: "call_1", name: "system", arguments: {command: "date"}}
+            ],
+            original_tool_calls: [
+              {"id" => "call_1", "type" => "function", "function" => {"name" => "system", "arguments" => "{\"command\":\"date\"}"}}
+            ]
+          })
+        end
+        before do
+          restored
+        end
+        let(:restored_message) { restored.messages.first }
+        let(:serialized_message) { JSON.parse(File.read(serialized)).fetch("messages").fetch(0) }
+        let(:tool_calls) do
+          restored_message.extra[:tool_calls].map do |tool|
+            tool.to_h.merge("arguments" => tool.arguments.to_h)
+          end
+        end
+        let(:original_tool_calls) do
+          restored_message.extra[:original_tool_calls].map do |tool|
+            tool.to_h.merge("function" => tool.function.to_h)
+          end
+        end
+
+        it "restores the message as a tool call" do
+          expect(restored_message.tool_call?).to eq(true)
+        end
+
+        it "serializes parsed tool calls under the tools key" do
+          expect(serialized_message.fetch("tools")).to eq([
+            {"id" => "call_1", "name" => "system", "arguments" => {"command" => "date"}}
+          ])
+        end
+
+        it "round-trips parsed tool calls" do
+          expect(tool_calls).to eq([
+            {"id" => "call_1", "name" => "system", "arguments" => {"command" => "date"}}
+          ])
+        end
+
+        it "round-trips original tool calls" do
+          expect(original_tool_calls).to eq([
+            {"id" => "call_1", "type" => "function", "function" => {"name" => "system", "arguments" => "{\"command\":\"date\"}"}}
+          ])
+        end
+      end
     end
   end
 

spec/message_spec.rb

@@ -38,7 +38,7 @@
     end
 
     it "normalizes tool calls to hashes" do
-      expect(message.to_h[:tool_calls]).to eq([{"id" => "call_1"}])
+      expect(message.to_h[:tools]).to eq([{"id" => "call_1"}])
     end
   end