Refactor timezone handling in mqtt datetime

[epenet]

Proposed change

Fixes race condition exposed in https://github.com/home-assistant/core/actions/runs/25005822536/job/73229778103?pr=162263

Needed for #162263

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[home-assistant]

Hey there @emontnemery, @jbouwh, @bdraco, mind taking a look at this pull request as it has been labeled with an integration (mqtt) you are listed as a code owner for? Thanks!

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[Copilot]

Pull request overview

Refactors how the MQTT datetime entity resolves a configured timezone, moving the async lookup out of _setup_from_config and into mqtt_async_added_to_hass to address a race condition seen in CI.

Changes:

• Store the configured timezone string during _setup_from_config instead of starting an async task.
• Resolve and validate the timezone asynchronously in mqtt_async_added_to_hass.

Stale