zephyr-cp: raise the rpi_pico UDC thread stack in debug builds

debug.conf bumps the DWC2 and nRF UDC thread stacks because verbose
logging with LOG_MODE_IMMEDIATE formats in-thread and needs more stack,
but the rpi_pico one was missed. It defaults to 512 bytes, which
overflows during USB enumeration on a Pico 2 W:

  ***** USAGE FAULT *****
    Stack overflow (context area not valid)
  >>> ZEPHYR FATAL ERROR 2: Stack overflow on CPU 0
  Current thread: 0x200034b0 (usbd@50110000)

The board dies partway through configuration, which looks like a bad USB
come-up rather than a stack problem. Independent of any Bluetooth work —
it affects any RP2040/RP2350 board built with DEBUG=1.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: tyeth

ports/zephyr-cp/debug.conf

@@ -35,6 +35,7 @@ CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE=4096
 CONFIG_MAIN_STACK_SIZE=32768
 CONFIG_ISR_STACK_SIZE=4096
 CONFIG_UDC_DWC2_STACK_SIZE=4096
+CONFIG_UDC_RPI_PICO_STACK_SIZE=2048
 CONFIG_UDC_NRF_THREAD_STACK_SIZE=4096
 CONFIG_USBD_THREAD_STACK_SIZE=4096
 CONFIG_USBD_MSC_STACK_SIZE=4096