added gemini api provider

Author: Nope1405

backend/app/services/gemini_provider.py

@@ -0,0 +1,71 @@
+
+import os
+import google.generativeai as genai
+from typing import Sequence, Any
+
+from .ai_service import AIProvider, Character, Message
+
+
+class GeminiAIProvider:
+    def __init__(self, api_key: str, model_name: str = 'gemini-2.5-flash'):
+        genai.configure(api_key=api_key)
+        self.model = genai.GenerativeModel(model_name)
+        self.model_name = model_name
+
+    def _format_history_for_gemini(self, history: Sequence[Message]) -> list[dict[str, Any]]:
+        gemini_history = []
+        for message in history:
+            role = 'user' if message.sender == 'user' else 'model'
+            gemini_history.append({'role': role, 'parts': [message.content]})
+        return gemini_history
+
+    def get_completion(self, character: Character, history: Sequence[Message]) -> str:
+        if not history:
+            return "Error: No conversation history."
+
+        last_message = history[-1]
+
+        if last_message.sender != 'user':
+             print(f"Warning: Gemini provider expects last message from user, but got {last_message.sender}.")
+             last_user_message_content = None
+             context_history_for_api = []
+             found_last_user = False
+
+             for msg in reversed(history):
+                  if msg.sender == 'user' and not found_last_user:
+                       last_user_message_content = msg.content
+                       found_last_user = True
+                  elif found_last_user:
+                       role = 'user' if msg.sender == 'user' else 'model'
+                       context_history_for_api.append({'role': role, 'parts': [msg.content]})
+
+             if not found_last_user or last_user_message_content is None:
+                 return "Error: No user message found in history for Gemini."
+
+             context_history_for_api.reverse()
+
+             formatted_context_history = context_history_for_api
+             current_user_prompt = last_user_message_content
+
+        else:
+            formatted_context_history = self._format_history_for_gemini(history[:-1])
+            current_user_prompt = last_message.content
+
+
+        system_instruction = character.system_prompt
+
+        try:
+            chat = self.model.start_chat(history=formatted_context_history,
+                                         system_instruction=system_instruction)
+            response = chat.send_message(current_user_prompt)
+            ai_response_text = response.text
+
+            if not ai_response_text:
+                 print("Warning: Gemini API returned no text.")
+                 return "I cannot generate a response for that request."
+
+            return ai_response_text
+
+        except Exception as e:
+            print(f"Error calling Gemini API: {e}")
+            return "Sorry, I encountered an error with the AI service."