feat(api): type AllowedUpdates as []UpdateType for compile-time typo safety

Both SetWebhookParams.AllowedUpdates and GetUpdatesParams.AllowedUpdates
(plus the WebhookInfo.AllowedUpdates field on the response side) were
typed []string, forcing callers to cast every typed constant:

  AllowedUpdates: []string{
      string(api.UpdateMessage),
      string(api.UpdateMyChatMember),
      ...
  }

Switch to []UpdateType so the same call site is typo-safe end-to-end:

  AllowedUpdates: []UpdateType{
      api.UpdateMessage,
      api.UpdateMyChatMember,
      ...
  }

Wire format is unchanged — UpdateType is type UpdateType string, marshals
identically as JSON strings. The MultipartFields()/runtime.go encoding
paths likewise continue to work via json.Marshal on the typed slice.

Implementation note: api/methods.gen.go and api/types.gen.go are
generated by cmd/genapi from internal/spec/api.json, where the field is
described as Array of String. The Telegram docs do not enumerate the
allowed_updates values inline, so the scraper cannot synthesise the
enum and UpdateType lives hand-curated in api/enums.go (see existing
doc comment there). The retype is therefore done as a small pinned
override inside cmd/genapi/emitter.go's goField — keyed on the wire
field name allowed_updates with elem type string — so the change
survives a future regeneration of the .gen.go files. transport/longpoll
drops the now-unnecessary []string conversion.

Backward incompatibility: callers passing untyped []string variables
will need to convert; callers using untyped string literals inside the
slice ARE fine because Go's untyped-literal rule auto-converts.

api/types.gen.go

@@ -91,7 +91,7 @@ type WebhookInfo struct {
 	// Optional. The maximum allowed number of simultaneous HTTPS connections to the webhook for update delivery
 	MaxConnections *int64 `json:"max_connections,omitempty"`
 	// Optional. A list of update types the bot is subscribed to. Defaults to all update types except chat_member
-	AllowedUpdates []string `json:"allowed_updates,omitempty"`
+	AllowedUpdates []UpdateType `json:"allowed_updates,omitempty"`
 }
 
 // This object represents a Telegram user or bot.