Refactor AI Assistant code

Related to #8 Refactor `AiAssistants.run` to return `ChatCompletionMessageParam[]` instead of modifying `updatedChatHistory` directly. * **src/AiAssistants.ts** - Modify `AiAssistants.run` to return a `ChatCompletionMessageParam[]`. - Update the function signature and implementation to reflect the return type. * **src/ai-assistants/postgresAssistant.ts** - Modify `postgresAssistant` to return a `ChatCompletionMessageParam[]`. - Update the function signature and implementation to reflect the return type. * **src/useAIAssistant.ts** - Update the call to `AiAssistants.run` to use the returned chat history. - Adjust the function implementation to handle the returned chat history. * **src/ai-assistants/postgresAssistant.test.ts** - Update tests to reflect the changes in `postgresAssistant`. - Adjust the test cases to handle the returned chat history. --- For more details, open the [Copilot Workspace session](https://copilot-workspace.githubnext.com/williamcotton/guish/issues/8?shareId=276841fd-2fe9-4709-95f9-d460dac43dbd).