Add in_waiting property to busio.UART

Add the in_waiting property to match CircuitPython's busio.UART API.
Returns the number of bytes available in the input buffer.

- RP2040 UART: wraps MicroPython's uart.any()
- busio.UART wrapper: checks for in_waiting first, falls back to
  any(), raises NotImplementedError if neither is available

Fixes #1046

Author: makermelissa-ai-assistant

src/adafruit_blinka/microcontroller/rp2040/uart.py

@@ -50,3 +50,8 @@ def readline(self):
     def write(self, buf):
         """Write to the UART from a buffer"""
         return self._uart.write(buf)
+
+    @property
+    def in_waiting(self):
+        """The number of bytes in the input buffer, available to be read"""
+        return self._uart.any()

src/busio.py

@@ -628,3 +628,14 @@ def readline(self):
     def write(self, buf):
         """Write to the UART from a buffer"""
         return self._uart.write(buf)
+
+    @property
+    def in_waiting(self):
+        """The number of bytes in the input buffer, available to be read"""
+        # Custom UART implementations (e.g. RP2040) that provide in_waiting directly
+        if hasattr(self._uart, "in_waiting"):
+            return self._uart.in_waiting
+        # MicroPython's machine.UART uses any() for the same purpose
+        if hasattr(self._uart, "any"):
+            return self._uart.any()
+        raise NotImplementedError("in_waiting not supported on this platform")