fix(robot-server): dont let /instruments block

It calls cache_instruments and that can block because it takes the
motion lock, but we really don't want that. We don't mind if
cache_instruments doesn't get called on the flex because it's sort of a
secondary functionality, so just bail early in this case.

Closes EXEC-298

api/src/opentrons/hardware_control/ot3api.py

@@ -653,12 +653,16 @@ def get_all_attached_instr(self) -> Dict[OT3Mount, Optional[InstrumentDict]]:
 
     # TODO (spp, 2023-01-31): add unit tests
     async def cache_instruments(
-        self, require: Optional[Dict[top_types.Mount, PipetteName]] = None
+        self,
+        require: Optional[Dict[top_types.Mount, PipetteName]] = None,
+        skip_if_would_block: bool = False,
     ) -> None:
         """
         Scan the attached instruments, take necessary configuration actions,
         and set up hardware controller internal state if necessary.
         """
+        if skip_if_would_block and self._motion_lock.locked():
+            return
         async with self._motion_lock:
             skip_configure = await self._cache_instruments(require)
             if not skip_configure or not self._configured_since_update:

robot-server/robot_server/instruments/router.py

@@ -216,7 +216,7 @@ async def _get_attached_instruments_ot3(
     hardware: OT3HardwareControlAPI,
 ) -> PydanticResponse[SimpleMultiBody[AttachedItem]]:
     # OT3
-    await hardware.cache_instruments()
+    await hardware.cache_instruments(skip_if_would_block=True)
     response_data = await _get_instrument_data(hardware)
     return await PydanticResponse.create(
         content=SimpleMultiBody.construct(