FIX: Mixtral models have system role support. (#703)

Using assistant role for system produces an error because
they expect alternating roles like user/assistant/user and so on.
Prompts cannot start with the assistant role.

lib/completions/dialects/dialect.rb

@@ -13,7 +13,6 @@ def all_dialects
             [
               DiscourseAi::Completions::Dialects::ChatGpt,
               DiscourseAi::Completions::Dialects::Gemini,
-              DiscourseAi::Completions::Dialects::Mistral,
               DiscourseAi::Completions::Dialects::Claude,
               DiscourseAi::Completions::Dialects::Command,
               DiscourseAi::Completions::Dialects::OpenAiCompatible,

spec/lib/completions/endpoints/hugging_face_spec.rb

@@ -94,7 +94,12 @@ def request_body(prompt, stream: false, tool_call: false)
   let(:hf_mock) { HuggingFaceMock.new(endpoint) }
 
   let(:compliance) do
-    EndpointsCompliance.new(self, endpoint, DiscourseAi::Completions::Dialects::Mistral, user)
+    EndpointsCompliance.new(
+      self,
+      endpoint,
+      DiscourseAi::Completions::Dialects::OpenAiCompatible,
+      user,
+    )
   end
 
   describe "#perform_completion!" do

spec/lib/completions/endpoints/vllm_spec.rb

@@ -69,10 +69,17 @@ def stub_streamed_response(prompt, deltas, tool_call: false)
   let(:anthropic_mock) { VllmMock.new(endpoint) }
 
   let(:compliance) do
-    EndpointsCompliance.new(self, endpoint, DiscourseAi::Completions::Dialects::Mistral, user)
+    EndpointsCompliance.new(
+      self,
+      endpoint,
+      DiscourseAi::Completions::Dialects::OpenAiCompatible,
+      user,
+    )
   end
 
-  let(:dialect) { DiscourseAi::Completions::Dialects::Mistral.new(generic_prompt, model_name) }
+  let(:dialect) do
+    DiscourseAi::Completions::Dialects::OpenAiCompatible.new(generic_prompt, model_name)
+  end
   let(:prompt) { dialect.translate }
 
   let(:request_body) { model.default_options.merge(messages: prompt).to_json }

spec/lib/completions/llm_spec.rb

@@ -3,7 +3,7 @@
 RSpec.describe DiscourseAi::Completions::Llm do
   subject(:llm) do
     described_class.new(
-      DiscourseAi::Completions::Dialects::Mistral,
+      DiscourseAi::Completions::Dialects::OpenAiCompatible,
       canned_response,
       "hugging_face:Upstage-Llama-2-*-instruct-v2",
       gateway: canned_response,