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/utils/json.go

@@ -1,3 +1,4 @@
+// Package utils provides utility functions for the Yandex Music API client.
 package utils
 
 import (
@@ -6,42 +7,49 @@ import (
 	"strings"
 )
 
-// JSONBackend defines the interface for JSON encoding/decoding
+// JSONBackend defines the interface for JSON encoding/decoding.
+// Implementations can use encoding/json, json-iterator, or other backends.
 type JSONBackend interface {
+	// Marshal encodes a Go value to JSON.
 	Marshal(v any) ([]byte, error)
+	// Unmarshal decodes JSON data into a Go value.
 	Unmarshal(data []byte, v any) error
 }
 
-// DefaultJSONBackend uses the standard library encoding/json
+// DefaultJSONBackend uses the standard library encoding/json.
 type DefaultJSONBackend struct{}
 
+// Marshal encodes a Go value to JSON using encoding/json.
 func (DefaultJSONBackend) Marshal(v any) ([]byte, error) {
 	return json.Marshal(v)
 }
 
+// Unmarshal decodes JSON data into a Go value using encoding/json.
 func (DefaultJSONBackend) Unmarshal(data []byte, v any) error {
 	return json.Unmarshal(data, v)
 }
 
-// GetDefaultJSONBackend returns the default JSON backend
+// GetDefaultJSONBackend returns the default JSON backend (encoding/json).
 func GetDefaultJSONBackend() JSONBackend {
 	return DefaultJSONBackend{}
 }
 
-// RawMessage is a raw encoded JSON value
+// RawMessage is a raw encoded JSON value.
+// It is an alias for encoding/json.RawMessage.
 type RawMessage = json.RawMessage
 
-// Marshal is a convenience function
+// Marshal is a convenience function that marshals a value using the default JSON backend.
 func Marshal(v any) ([]byte, error) {
 	return GetDefaultJSONBackend().Marshal(v)
 }
 
-// Unmarshal is a convenience function
+// Unmarshal is a convenience function that unmarshals data using the default JSON backend.
 func Unmarshal(data []byte, v any) error {
 	return GetDefaultJSONBackend().Unmarshal(data, v)
 }
 
-// ToSnakeCase converts camelCase to snake_case
+// ToSnakeCase converts camelCase to snake_case.
+// For example, "camelCase" becomes "camel_case".
 func ToSnakeCase(s string) string {
 	if s == "" {
 		return s
@@ -60,7 +68,8 @@ func ToSnakeCase(s string) string {
 	return result.String()
 }
 
-// ToCamelCase converts snake_case to camelCase
+// ToCamelCase converts snake_case to camelCase.
+// For example, "snake_case" becomes "snakeCase".
 func ToCamelCase(s string) string {
 	if s == "" {
 		return s
@@ -79,7 +88,7 @@ func ToCamelCase(s string) string {
 	return result.String()
 }
 
-// MapKeysToSnakeCase converts all keys in a map to snake_case
+// MapKeysToSnakeCase converts all keys in a map to snake_case.
 func MapKeysToSnakeCase(m map[string]any) map[string]any {
 	result := make(map[string]any, len(m))
 	for k, v := range m {
@@ -88,7 +97,7 @@ func MapKeysToSnakeCase(m map[string]any) map[string]any {
 	return result
 }
 
-// MapKeysToCamelCase converts all keys in a map to camelCase
+// MapKeysToCamelCase converts all keys in a map to camelCase.
 func MapKeysToCamelCase(m map[string]any) map[string]any {
 	result := make(map[string]any, len(m))
 	for k, v := range m {
@@ -97,7 +106,8 @@ func MapKeysToCamelCase(m map[string]any) map[string]any {
 	return result
 }
 
-// StructToMap converts a struct to a map using json tags
+// StructToMap converts a struct to a map using json tags.
+// The forRequest parameter is currently unused but reserved for future use.
 func StructToMap(v any, forRequest bool) (map[string]any, error) {
 	data, err := Marshal(v)
 	if err != nil {
@@ -110,7 +120,8 @@ func StructToMap(v any, forRequest bool) (map[string]any, error) {
 	return result, nil
 }
 
-// GetJSONTag returns the json tag for a struct field
+// GetJSONTag returns the json tag name for a struct field at the given index.
+// It returns an empty string if the field has no json tag or the tag is "-".
 func GetJSONTag(t reflect.Type, fieldIndex int) string {
 	if fieldIndex >= t.NumField() {
 		return ""