Stage 16: connector test-env via UseTestEnvironment; pin it in the test contour

- bot.New now selects Telegram's test environment with the library's native
  tgbot.UseTestEnvironment() instead of a token += "/test" hack (functionally
  identical URL /bot<token>/test/METHOD, but idiomatic) + a bot test asserting
  the getMe path for both test and prod.
- ci.yaml pins TELEGRAM_TEST_ENV=true for the test contour (it IS the test
  environment) instead of a TEST_TELEGRAM_TEST_ENV variable: removes the
  confusing double-TEST, telegram-specific, prefixed operator knob and the
  secret-vs-variable footgun. Prod (Stage 17) leaves it false.
- deploy/README.md + PLAN.md updated.

platform/telegram/internal/bot/bot.go

@@ -43,21 +43,18 @@ func New(cfg Config, log *zap.Logger) (*Bot, error) {
 	}
 	t := &Bot{miniAppURL: cfg.MiniAppURL, log: log}
 
-	token := cfg.Token
-	if cfg.TestEnv {
-		// The Bot API test environment lives under /bot<token>/test/METHOD; the
-		// client builds <host>/bot<token>/<method>, so suffixing the token with
-		// "/test" injects the test segment without a custom host.
-		token += "/test"
-	}
 	opts := []tgbot.Option{
 		tgbot.WithDefaultHandler(t.handleStart),
 		tgbot.WithMessageTextHandler("/start", tgbot.MatchTypePrefix, t.handleStart),
 	}
+	if cfg.TestEnv {
+		// Route to the Bot API test environment (.../bot<token>/test/METHOD).
+		opts = append(opts, tgbot.UseTestEnvironment())
+	}
 	if cfg.APIBaseURL != "" {
 		opts = append(opts, tgbot.WithServerURL(cfg.APIBaseURL))
 	}
-	api, err := tgbot.New(token, opts...)
+	api, err := tgbot.New(cfg.Token, opts...)
 	if err != nil {
 		return nil, err
 	}