Add comprehensive godoc documentation to all exported symbols

- Document all exported types, functions, methods, and constants across 52 files
- Add package-level doc comments to client, models, utils, and ynison packages
- Document HTTP method and endpoint path for each API client method
- Document model struct fields and DeJSON/DeList/ToJSON/ToMap methods
- Fix duplicate package doc comment in internal/models/base.go

Co-Authored-By: Claude Code <noreply@anthropic.com>

internal/models/client.go

@@ -1,19 +1,28 @@
+// Package models provides data models for the Yandex Music API.
 package models
 
 import (
 	"context"
 )
 
-// ClientInterface defines the minimal client interface needed by models
+// ClientInterface defines the minimal client interface needed by models.
+// Models use this to access the underlying request handler.
 type ClientInterface interface {
+	// GetRequest returns the underlying request handler.
 	GetRequest() RequestInterface
 }
 
-// RequestInterface defines the minimal request interface needed by models
+// RequestInterface defines the minimal request interface needed by models.
+// It provides methods for making HTTP requests to the Yandex Music API.
 type RequestInterface interface {
+	// Get performs a GET request to the given path with query parameters.
 	Get(ctx context.Context, path string, params map[string]string, result any) error
+	// Post performs a POST request to the given path with a JSON body.
 	Post(ctx context.Context, path string, body any, result any) error
+	// Put performs a PUT request to the given path with a JSON body.
 	Put(ctx context.Context, path string, body any, result any) error
+	// Delete performs a DELETE request to the given path with an optional JSON body.
 	Delete(ctx context.Context, path string, body any, result any) error
+	// Download downloads a file from the given URL and returns the raw response body.
 	Download(ctx context.Context, url string) ([]byte, error)
 }