Correct the type for RunMode.build_mode

This is a bit of an ugly dance, but checks the type properly at
run-time and also convinces Pyright about the existence and type of
RunMode.build_mode.

Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>

Author: rswarbrick

src/dvsim/modes.py

@@ -301,14 +301,21 @@ def __init__(self, rdict) -> None:
         self.run_opts = []
         self.uvm_test = ""
         self.uvm_test_seq = ""
-        self.build_mode = ""
         self.run_timeout_mins = None
         self.run_timeout_multiplier = None
         self.sw_images = []
         self.sw_build_device = ""
         self.sw_build_opts = []
 
         super().__init__("run mode", rdict)
+
+        # The base class constructor should have provided a BuildMode object in
+        # self.build_mode. Check this at run-time. The odd
+        # build_mode=build_mode line is to teach the pyright about the field.
+        if not isinstance(self.build_mode, BuildMode):
+            raise TypeError("build_mode not computed by constructor")
+        self.build_mode: BuildMode = self.build_mode
+
         self.en_run_modes = list(set(self.en_run_modes))
 
     def get_sub_modes(self) -> list[str]: