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/client/artists.go

@@ -1,3 +1,4 @@
+// Package client provides methods for interacting with Yandex Music API endpoints.
 package client
 
 import (
@@ -8,7 +9,8 @@ import (
 	"gitea.mrixs.me/Mrixs/yandex-music-api-go/internal/utils"
 )
 
-// GetArtists gets artists by IDs
+// GetArtists gets artists by their IDs (GET /tracks?artist-ids=...).
+// Returns a slice of Artist pointers, or nil if no IDs are provided.
 func (c *Client) GetArtists(ctx context.Context, artistIDs []string) ([]*models.Artist, error) {
 	if len(artistIDs) == 0 {
 		return nil, nil
@@ -31,7 +33,8 @@ func (c *Client) GetArtists(ctx context.Context, artistIDs []string) ([]*models.
 	return artists, nil
 }
 
-// GetArtist gets a single artist by ID
+// GetArtist gets a single artist by ID using GetArtists.
+// Returns an error if no artist is found with the given ID.
 func (c *Client) GetArtist(ctx context.Context, artistID string) (*models.Artist, error) {
 	artists, err := c.GetArtists(ctx, []string{artistID})
 	if err != nil {
@@ -43,8 +46,8 @@ func (c *Client) GetArtist(ctx context.Context, artistID string) (*models.Artist
 	return artists[0], nil
 }
 
-// GetArtistAlbums gets albums by an artist
-// GET /artists/{artist_id}/direct-albums
+// GetArtistAlbums gets albums by an artist (GET /artists/{artist_id}/direct-albums).
+// The page and pageSize parameters control pagination.
 func (c *Client) GetArtistAlbums(ctx context.Context, artistID string, page, pageSize int) (*models.ArtistAlbums, error) {
 	params := map[string]string{
 		"page":      fmt.Sprintf("%d", page),
@@ -59,8 +62,8 @@ func (c *Client) GetArtistAlbums(ctx context.Context, artistID string, page, pag
 	return &result, nil
 }
 
-// GetArtistTracks gets tracks by an artist
-// GET /artists/{artist_id}/tracks
+// GetArtistTracks gets tracks by an artist (GET /artists/{artist_id}/tracks).
+// The page and pageSize parameters control pagination.
 func (c *Client) GetArtistTracks(ctx context.Context, artistID string, page, pageSize int) (*models.ArtistTracks, error) {
 	params := map[string]string{
 		"page":      fmt.Sprintf("%d", page),