Refactor Bot Lifecycle and Event Synchronization

[PenguinBoi12]

This PR refactors the Bot lifecycle to clearly separate synchronization and readiness:

Previously, scheduled tasks or extensions could attempt to access bot state before rooms were available, leading to crashes. The new lifecycle ensures predictable ordering: sync → ready → scheduler, improving reliability and making asynchronous tests non-blocking.

• Adds _synced (asyncio.Event) to track when the client has completed initial syncing.
• _wait_until_synced now waits for the first sync before firing on_ready.
• on_ready is guaranteed to run once, after the bot is fully synchronized, before scheduled tasks are started.
• Scheduler start is now explicitly after on_ready, ensuring jobs can safely access bot state and rooms.
• get_room updated to return None instead of raising, preventing crashes during startup.
• Tests updated to avoid blocking on sync_forever, by setting _synced manually and mocking long-running tasks.

[chrisdedman]

Nice refactor. LGTM

[chrisdedman]

LGTM