fix(webhook): return 500 on handler failure to enable Telegram retries

[Dhereal1]

Problem

Webhook always returned HTTP 200 immediately while processing updates asynchronously.
If handleUpdate failed, the error was logged but Telegram considered the update delivered successfully, resulting in silent message drops.

Fix

• Await bot.handleUpdate(...) directly in the request lifecycle
• Remove reliance on waitUntil for core update processing
• Return HTTP 500 on failure so Telegram retries delivery
• Keep per-chat serialization to preserve message order

Result

• Failed updates are retried by Telegram
• Eliminates silent message drops
• Improves reliability under transient errors

Testing

• Verified locally with polling mode
• /start responds correctly
• Simulated failure returns 500 and does not silently succeed

[staindart]

This is likely not needed as we should feedback Telegram with 200 for the bot correct work.