Fix API parameter handling and improve onCommand handling

[antonkomarev]

Breaking Changes

• Rename HttpClient::post() → postMultipart() for clarity
• Make Keyboard::$rows private (was accidentally public)

Bug Fixes

• Fix json_encode of nullable params (forwardMsgId, inlineKeyboardMarkup) — previously encoded null as "null" string instead of being filtered out
• Remove !== 'null' workaround in filterParams, now redundant after null-check fix
• Fix BotEventListener command matching — prevent /start from matching /startGame (now requires exact match, space, or @ after command)

Features

• Add cursor parameter to ChatsApi::getMembers()
• Add onException callback to BotEventListener::listen() — allows handling exceptions from event handlers without stopping the polling loop
• Prevent duplicate command handler registration in BotEventListener::onCommand() — throws InvalidArgumentException if already registered

Tests

• Add unit tests for MessagesApi, ChatsApi, EventsApi — covers parameter formation, fileId/filePath branching, parseMode defaults and overrides
• Add tests for onException behavior in BotEventListener
• Extract shared HttpClientSpy test helper to test/Spy/