Respect user-defined CDC string descriptor

[verylowfreq]

This PR allows user sketches to set a custom string descriptor for the CDC-ACM interface.

What changed

Previously, Adafruit_USBD_CDC::begin() always set the CDC interface string descriptor to "TinyUSB Serial". This prevented user sketches from overriding the CDC interface string with setStringDescriptor().

This patch changes the behavior to:

1. Check whether _strid has already been set by the user sketch.
2. If _strid is still 0, use the default string descriptor "TinyUSB Serial".
3. If the user sketch already called setStringDescriptor(), keep that user-defined string.

Use case

On some boards supported by Adafruit TinyUSB, Serial.begin() may be called before setup(). In that case, a sketch can clear and rebuild the USB configuration with a custom CDC interface string:

void setup() {
  // Clear all configurations including default Serial interface
  TinyUSBDevice.clearConfiguration();

  // Set custom string for CDC interface
  Serial.setStringDescriptor("My Special Serial Port");
  // For re-building descriptor
  TinyUSBDevice.addInterface(Serial);
  // The custom string descriptor is preserved.
  Serial.begin(115200);
}

This makes it possible for the custom CDC-ACM interface name to appear on the host instead of always using "TinyUSB Serial".

[Copilot]

Pull request overview

This PR updates the CDC-ACM interface initialization so a user sketch can set a custom CDC interface string descriptor via setStringDescriptor() without it being overwritten during Adafruit_USBD_CDC::begin().

Changes:

• Only applies the default CDC interface string (“TinyUSB Serial”) if _strid has not already been set by the user.

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