[cmd3] Enforce command lifetimes across all opmode and command scopes (#8705)

Commands are no longer able to outlive their schedule-site's scope,
regardless of how they were scheduled (set as a default command, bound
to a trigger, or manually scheduled)

As a consequence, default commands need better tracking so the default
command setting can be released when their scope exits and the next-most
appropriate default command can be rescheduled (eg, an opmode sets a
default command, then the globally-scoped default is restored when the
opmode exits). Some complexity is required here to make it work well for
edge cases.

Like `schedule()`, `setDefaultCommand()` will immediately start the new
default command if called inside of another command to avoid 1-loop
delays. However, this does not apply when called by the _current_
default command, as it would result in attempting to cancel the default
command while it's mounted (which is impossible and would throw an
exception)

```java
class Robot extends OpModeRobot {
  final Drive drive = new Drive();
  final CommandXboxController controller = new CommandXboxController(1);

  public Robot() {
    // global default command, active unless overridden in an opmode or command
    drive.setDefaultCommand(drive.stop());

    // global trigger binding, always active
    controller.rightBumper().onTrue(drive.setX());
  }
}

@Teleop
class ExampleOpMode extends PeriodicOpMode {
  public ExampleOpMode(Robot robot) {
    // opmode-specific default command
    robot.drive.setDefaultCommand(robot.drive.operatorControl(robot.controller));

    // opmode-specific binding
    robot.controller.leftBumper().whileTrue(robot.drive.stop());

    // opmode-specific binding that takes precedence over the global binding
    // because it happens last; it "wins out" over the `setX()` binding
    robot.controller.rightBumper().onTrue(robot.drive.selfTest());
  }

  @Override
  public void periodic() {
    Scheduler.getDefault().run();
  }
}
```

wpilibj/src/test/java/org/wpilib/driverstation/DriverStationTest.java

@@ -8,6 +8,8 @@ import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.params.provider.Arguments.arguments;
 
 import java.util.stream.Stream;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
 import org.junit.jupiter.params.provider.MethodSource;
@@ -26,6 +28,34 @@ class DriverStationTest {
     assertEquals(expected, DriverStation.isJoystickConnected(1));
   }
 
+  @Test
+  void getOpmodeIdReturnsZeroUntilUserProgramStarts() {
+    DriverStationSim.setOpMode(0x1234);
+    DriverStationSim.notifyNewData();
+    assertEquals(0, DriverStation.getOpModeId());
+
+    DriverStation.observeUserProgramStarting();
+    // need to manually mask because the upper eight bits include robot mode information
+    assertEquals(0x1234, DriverStation.getOpModeId() & 0xFFFF);
+  }
+
+  @Test
+  void getOpmodeReturnsEmptyStringUntilUserProgramStarts() {
+    DriverStationSim.setOpMode(0x1234);
+    DriverStationSim.notifyNewData();
+    assertEquals("", DriverStation.getOpMode());
+
+    DriverStation.observeUserProgramStarting();
+    // in Sim, the opmode string is just the stringified version of the opmode i64 "<0000...0000>"
+    // we need to parse the string to get the
+    // need to manually mask because the upper eight bits include robot mode information
+    String opmodeName = DriverStation.getOpMode();
+    assertEquals(
+        "0x1234",
+        String.format(
+            "0x%x", Long.parseLong(opmodeName.substring(1, opmodeName.length() - 1)) & 0xFFFF));
+  }
+
   static Stream<Arguments> isConnectedProvider() {
     return Stream.of(
         arguments(0, 0, 0, false),
@@ -52,4 +82,12 @@ class DriverStationTest {
         arguments(true, true, false),
         arguments(true, false, false));
   }
+
+  @AfterEach
+  @SuppressWarnings("PMD.AvoidAccessibilityAlteration")
+  void resetUserProgramFlag() throws ReflectiveOperationException {
+    var field = DriverStation.class.getDeclaredField("m_userProgramStarted");
+    field.setAccessible(true);
+    field.set(null, false);
+  }
 }