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.

google.go

@@ -1,4 +1,4 @@
-package go_llm
+package llm
 
 import (
 	"context"
@@ -11,22 +11,24 @@ import (
 	"google.golang.org/genai"
 )
 
-type google struct {
+type googleImpl struct {
 	key   string
 	model string
 }
 
-func (g google) ModelVersion(modelVersion string) (ChatCompletion, error) {
+var _ LLM = googleImpl{}
+
+func (g googleImpl) ModelVersion(modelVersion string) (ChatCompletion, error) {
 	g.model = modelVersion
 
 	return g, nil
 }
 
-func (g google) requestToContents(in Request) ([]*genai.Content, *genai.GenerateContentConfig) {
+func (g googleImpl) requestToContents(in Request) ([]*genai.Content, *genai.GenerateContentConfig) {
 	var contents []*genai.Content
 	var cfg genai.GenerateContentConfig
 
-	for _, tool := range in.Toolbox.functions {
+	for _, tool := range in.Toolbox.Functions() {
 		cfg.Tools = append(cfg.Tools, &genai.Tool{
 			FunctionDeclarations: []*genai.FunctionDeclaration{
 				{
@@ -101,7 +103,7 @@ func (g google) requestToContents(in Request) ([]*genai.Content, *genai.Generate
 	return contents, &cfg
 }
 
-func (g google) responseToLLMResponse(in *genai.GenerateContentResponse) (Response, error) {
+func (g googleImpl) responseToLLMResponse(in *genai.GenerateContentResponse) (Response, error) {
 	res := Response{}
 
 	for _, c := range in.Candidates {
@@ -142,7 +144,7 @@ func (g google) responseToLLMResponse(in *genai.GenerateContentResponse) (Respon
 	return res, nil
 }
 
-func (g google) ChatComplete(ctx context.Context, req Request) (Response, error) {
+func (g googleImpl) ChatComplete(ctx context.Context, req Request) (Response, error) {
 	cl, err := genai.NewClient(ctx, &genai.ClientConfig{
 		APIKey:  g.key,
 		Backend: genai.BackendGeminiAPI,