fix(openthread): restore ot_network_auto_start() call in ot_br example (IDFGH-17984)

[cnicc]

Description

On cold boot, ot state stays disabled in the ot_br example even when CONFIG_OPENTHREAD_NETWORK_AUTO_START=y and a valid Active Dataset is already persisted in NVS. Wi-Fi connects and the border-routing services start correctly, but the Thread interface itself is never enabled: the device only attaches after manually re-entering ot dataset ... commit active + ot ifconfig up + ot thread start over serial on every reboot.

esp_openthread_border_router_start() only brings up Wi-Fi and the border-routing services, but it never calls otIp6SetEnabled/otThreadSetEnabled.
The call that does this, ot_network_auto_start(), was dropped from ot_br's app_main() in 0697295 ("optimized the autostart macro"), while CONFIG_OPENTHREAD_BORDER_ROUTER_AUTO_START and CONFIG_OPENTHREAD_NETWORK_AUTO_START were being consolidated into a single Kconfig option. ot_cli's app_main() was not touched by that commit and still calls ot_network_auto_start().

This PR restores the ot_network_auto_start() call in ot_br's app_main(),
matching ot_cli. No Kconfig/sdkconfig changes are needed.

Related

No existing issue or PR found for this.
Regression introduced in 0697295.

Testing

Tested on an ESP32-C6, standalone single-chip Thread Border Router, ot_br example, ESP-IDF v6.0.2.
Before, ot dataset active -x confirms the Active Dataset persists correctly, but ot state output was disabled after a cold boot.
After the fix, the device auto-attaches on boot with no serial commands.

No automated test added: verifying ot state after a cold boot requires hardware-in-the-loop testing, which the existing sdkconfig.ci.br_autostart CI config doesn't cover.

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Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.

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Note

Low Risk
Single guarded call in an example app that restores prior autostart behavior; no Kconfig or core stack API changes.

Overview
Restores ot_network_auto_start() in the ot_br example’s app_main() when CONFIG_OPENTHREAD_NETWORK_AUTO_START is enabled, aligning startup with ot_cli.

Border-router startup (esp_openthread_border_router_start()) only brings up Wi‑Fi and border-routing services; it does not enable the Thread stack from a persisted Active Dataset. Without this call, cold boot leaves ot state disabled until manual CLI steps, even when auto-start is configured in Kconfig.

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