Anthropic: Fix system prompt (use plain text instead of serialized JSON) (crmne#302)

## What this does

When migrating from
[ruby-openai](https://github.com/alexrudall/ruby-openai), I had some
issues getting the same responses in my Anthropic test suite.

After some digging, I observed that the Anthropic requests send the
`system context` as serialized JSON instead of a plain string like
described in the [API
reference](https://docs.anthropic.com/en/api/messages#body-system):

```ruby
{
  :system => "{type:\n        \"text\", text: \"You must include the exact phrase \\\"XKCD7392\\\" somewhere\n        in your response.\"}",
  [...]
}
```

instead of :  

```ruby
{
  :system => "You must include the exact phrase \"XKCD7392\" somewhere in your response.",
  [...]
}
```

It works quite well (the model still understands it) but it uses more
tokens than needed. It could also mislead the model in interpreting the
system prompt.

This PR fixed it. I also took the initiative to make the temperature an
optional parameter ([just like with
OpenAI](https://github.com/crmne/ruby_llm/blob/main/lib/ruby_llm/providers/openai/chat.rb#L21-L22)).
I hope it's not too much for a single PR, but since I was already
re-recording the cassettes, I figured it would be easier.

I'm sorry but I don't have any API key for Bedrock/OpenRouter. I only
recorded the main Anthropic cassettes.

## Type of change

- [x] Bug fix
- [ ] New feature
- [ ] Breaking change
- [ ] Documentation
- [ ] Performance improvement

## Scope check

- [x] I read the [Contributing
Guide](https://github.com/crmne/ruby_llm/blob/main/CONTRIBUTING.md)
- [x] This aligns with RubyLLM's focus on **LLM communication**
- [x] This isn't application-specific logic that belongs in user code
- [x] This benefits most users, not just my specific use case

## Quality check

- [x] I ran `overcommit --install` and all hooks pass
- [x] I tested my changes thoroughly
- [ ] I updated documentation if needed
- [x] I didn't modify auto-generated files manually (`models.json`,
`aliases.json`)

## API changes

- [ ] Breaking change
- [ ] New public methods/classes
- [ ] Changed method signatures
- [ ] No API changes

## Related issues

<!-- Link issues: "Fixes crmne#123" or "Related to crmne#123" -->

---------

Co-authored-by: Carmine Paolino <[email protected]>

Author: 2 people

lib/ruby_llm/providers/anthropic/chat.rb

@@ -15,8 +15,8 @@ def render_payload(messages, tools:, temperature:, model:, stream: false, schema
           system_messages, chat_messages = separate_messages(messages)
           system_content = build_system_content(system_messages)
 
-          build_base_payload(chat_messages, temperature, model, stream).tap do |payload|
-            add_optional_fields(payload, system_content:, tools:)
+          build_base_payload(chat_messages, model, stream).tap do |payload|
+            add_optional_fields(payload, system_content:, tools:, temperature:)
           end
         end
 
@@ -32,22 +32,22 @@ def build_system_content(system_messages)
             )
           end
 
-          system_messages.map { |msg| format_message(msg)[:content] }.join("\n\n")
+          system_messages.map(&:content).join("\n\n")
         end
 
-        def build_base_payload(chat_messages, temperature, model, stream)
+        def build_base_payload(chat_messages, model, stream)
           {
             model: model,
             messages: chat_messages.map { |msg| format_message(msg) },
-            temperature: temperature,
             stream: stream,
             max_tokens: RubyLLM.models.find(model)&.max_tokens || 4096
           }
         end
 
-        def add_optional_fields(payload, system_content:, tools:)
+        def add_optional_fields(payload, system_content:, tools:, temperature:)
           payload[:tools] = tools.values.map { |t| Tools.function_for(t) } if tools.any?
           payload[:system] = system_content unless system_content.empty?
+          payload[:temperature] = temperature unless temperature.nil?
         end
 
         def parse_completion_response(response)

lib/ruby_llm/providers/bedrock/chat.rb

@@ -46,16 +46,15 @@ def render_payload(messages, tools:, temperature:, model:, stream: false, schema
           system_messages, chat_messages = Anthropic::Chat.separate_messages(messages)
           system_content = Anthropic::Chat.build_system_content(system_messages)
 
-          build_base_payload(chat_messages, temperature, model).tap do |payload|
-            Anthropic::Chat.add_optional_fields(payload, system_content:, tools:)
+          build_base_payload(chat_messages, model).tap do |payload|
+            Anthropic::Chat.add_optional_fields(payload, system_content:, tools:, temperature:)
           end
         end
 
-        def build_base_payload(chat_messages, temperature, model)
+        def build_base_payload(chat_messages, model)
           {
             anthropic_version: 'bedrock-2023-05-31',
             messages: chat_messages.map { |msg| format_message(msg) },
-            temperature: temperature,
             max_tokens: RubyLLM.models.find(model)&.max_tokens || 4096
           }
         end