Fix nRF52 RS232 bridge baud initialization

[axhoff]

Summary

• restart an already-active Adafruit nRF52 UART before initializing the RS232 bridge
• reapply the bridge pins after shutdown, then use the existing common begin(bridge_baud) path
• leave unopened UARTs and all non-nRF52 platforms on their existing initialization paths

The pinned Adafruit nRF52 core ignores begin() while a Uart is already active. That can leave a bridge configured for 115200 baud running at an earlier component's baud rate. The active-state check is intentional: this core's end() does not guard an unopened UART, so calling it unconditionally is unsafe.

The selected UART belongs exclusively to the bridge after initialization; configurations that would otherwise share it with GPS must disable GPS.

Validation

• pio test -e native (13 tests passed)
• pio run -e RAK_4631_repeater_bridge_rs232_serial1
• pio run -e RAK_4631_repeater_bridge_rs232_serial2
• pio run -e Heltec_t114_without_display_repeater_bridge_rs232
• pio run -e ProMicro_repeater_bridge_rs232_serial1
• pio run -e Heltec_t096_repeater_bridge_rs232
• adversarial implementation review: no correctness blockers found

The reported RAK3401 configuration has no official PlatformIO RS232 environment in this repository; its register-level and end-to-end hardware validation is documented in the linked issue.

Fixes #3011