Refactor: modularize and streamline LLM providers and utility functions

- Migrate `compress_image.go` to `internal/imageutil` for better encapsulation.
- Reorganize LLM provider implementations into distinct packages (`google`, `openai`, and `anthropic`).
- Replace `go_llm` package name with `llm`.
- Refactor internal APIs for improved clarity, including renaming `anthropic` to `anthropicImpl` and `google` to `googleImpl`.
- Add helper methods and restructure message handling for better separation of concerns.

response.go

@@ -1,9 +1,6 @@
-package go_llm
-
-import (
-	"github.com/openai/openai-go"
-)
+package llm
 
+// ResponseChoice represents a single choice in a response.
 type ResponseChoice struct {
 	Index   int
 	Role    Role
@@ -32,36 +29,6 @@ func (r ResponseChoice) toRaw() map[string]any {
 	return res
 }
 
-func (r ResponseChoice) toChatCompletionMessages(_ string) []openai.ChatCompletionMessageParamUnion {
-	var as openai.ChatCompletionAssistantMessageParam
-
-	if r.Name != "" {
-		as.Name = openai.String(r.Name)
-	}
-	if r.Refusal != "" {
-		as.Refusal = openai.String(r.Refusal)
-	}
-
-	if r.Content != "" {
-		as.Content.OfString = openai.String(r.Content)
-	}
-
-	for _, call := range r.Calls {
-		as.ToolCalls = append(as.ToolCalls, openai.ChatCompletionMessageToolCallParam{
-			ID: call.ID,
-			Function: openai.ChatCompletionMessageToolCallFunctionParam{
-				Name:      call.FunctionCall.Name,
-				Arguments: call.FunctionCall.Arguments,
-			},
-		})
-	}
-	return []openai.ChatCompletionMessageParamUnion{
-		{
-			OfAssistant: &as,
-		},
-	}
-}
-
 func (r ResponseChoice) toInput() []Input {
 	var res []Input
 
@@ -79,6 +46,7 @@ func (r ResponseChoice) toInput() []Input {
 	return res
 }
 
+// Response represents a response from a language model.
 type Response struct {
 	Choices []ResponseChoice
 }