tdh8.py: allow pasting of airband frequencies - (3 of 3) - fixes #11363

Add support to mark mode as immutable so it doesn't get blocked, and
let the radio override it during set.

chirp/drivers/tdh8.py

@@ -1005,6 +1005,7 @@ class TDH8(chirp_common.CloneModeRadio):
     ident_mode = b'P31183\xff\xff'
     BAUD_RATE = 38400
     NEEDS_COMPAT_SERIAL = False
+    MODES = ["FM", "NFM"]
     _memsize = 0x1eef
     _ranges_main = [(0x0000, 0x1eef)]
     _idents = [TD_H8]
@@ -1024,6 +1025,22 @@ class TDH8(chirp_common.CloneModeRadio):
                  chirp_common.PowerLevel("Mid",  watts=4.00),
                  chirp_common.PowerLevel("High", watts=8.00)]
 
+    def check_set_memory_immutable_policy(self, existing, new):
+        if (AIRBAND[0] <= new.freq <= AIRBAND[1] and
+                new.mode == 'AM'):
+            # This is valid, so mark mode as immutable so it doesn't get
+            # blocked, and let the radio override it during set.
+            new.immutable.append('mode')
+            existing.immutable = []
+        elif existing.mode == 'AM' and new.mode in self.MODES:
+            # If we're going from a forced-AM channel to some valid one,
+            # clear immutable so we allow the change.
+            try:
+                existing.immutable.remove('mode')
+            except ValueError:
+                pass
+        super().check_set_memory_immutable_policy(existing, new)
+
     @classmethod
     def detect_from_serial(cls, pipe):
         ident = _do_ident(pipe, cls._idents[0])
@@ -1076,7 +1093,7 @@ def get_features(self):
             "DTCS->DTCS"]
         rf.valid_power_levels = [x for x in self._tx_power if x]
         rf.valid_duplexes = ["", "-", "+", "split", "off"]
-        rf.valid_modes = ["FM", "NFM"]
+        rf.valid_modes = self.MODES
         rf.valid_tuning_steps = STEPS
 
         rf.valid_bands = self._txbands + self._rxbands
@@ -1265,6 +1282,10 @@ def get_memory(self, number):
             mem.duplex = 'off'
             mem.immutable.append('duplex')
         if in_range(mem.freq, [AIRBAND]):
+            # NOTE: AM is not in valid_modes because you can't arbitrarily
+            # enable it on this radio. However, we can expose it as immutable
+            # which will display properly in the UI and not allow the user
+            # to change those channels to FM.
             mem.mode = 'AM'
             mem.immutable.append('mode')