fix(gemini): map chat roles to Gemini user/model in convertMessages

CHANGES
- Map assistant role to model per Gemini constraints
- Map system, developer, function, tool roles to user
- Default unrecognized roles to user to preserve instruction context
- Add unit test validating convertMessages role mapping logic
- Import chat package in tests for role constants

internal/plugins/ai/gemini/gemini.go

@@ -336,6 +336,19 @@ func (o *Client) convertMessages(msgs []*chat.ChatCompletionMessage) []*genai.Co
 	for _, msg := range msgs {
 		content := &genai.Content{Parts: []*genai.Part{}}
 
+		switch msg.Role {
+		case chat.ChatMessageRoleAssistant:
+			content.Role = "model"
+		case chat.ChatMessageRoleUser:
+			content.Role = "user"
+		case chat.ChatMessageRoleSystem, chat.ChatMessageRoleDeveloper, chat.ChatMessageRoleFunction, chat.ChatMessageRoleTool:
+			// Gemini's API only accepts "user" and "model" roles.
+			// Map all other roles to "user" to preserve instruction context.
+			content.Role = "user"
+		default:
+			content.Role = "user"
+		}
+
 		if msg.Content != "" {
 			content.Parts = append(content.Parts, &genai.Part{Text: msg.Content})
 		}

internal/plugins/ai/gemini/gemini_test.go

@@ -4,6 +4,8 @@ import (
 	"testing"
 
 	"google.golang.org/genai"
+
+	"github.com/danielmiessler/fabric/internal/chat"
 )
 
 // Test buildModelNameFull method
@@ -51,6 +53,30 @@ func TestExtractTextFromResponse(t *testing.T) {
 	}
 }
 
+// Test convertMessages handles role mapping correctly
+func TestConvertMessagesRoles(t *testing.T) {
+	client := &Client{}
+	msgs := []*chat.ChatCompletionMessage{
+		{Role: chat.ChatMessageRoleUser, Content: "user"},
+		{Role: chat.ChatMessageRoleAssistant, Content: "assistant"},
+		{Role: chat.ChatMessageRoleSystem, Content: "system"},
+	}
+
+	contents := client.convertMessages(msgs)
+
+	expected := []string{"user", "model", "user"}
+
+	if len(contents) != len(expected) {
+		t.Fatalf("expected %d contents, got %d", len(expected), len(contents))
+	}
+
+	for i, c := range contents {
+		if c.Role != expected[i] {
+			t.Errorf("content %d expected role %s, got %s", i, expected[i], c.Role)
+		}
+	}
+}
+
 // Test isTTSModel method
 func TestIsTTSModel(t *testing.T) {
 	client := &Client{}