Fix DC motor Vmax voltage limiting in voltage input mode (#3489)

[Adityakk9031]

Fixes #3489

Problem

When a <dcmotor> is configured with Vmax in voltage-commanded mode (input="voltage") without an inductance state:

1. mjGAIN_DCMOTOR skipped dcmotorVoltage() due to checking ctrlspec != mjINPUT_VOLTAGE, leaving ctrl[uadr] unclamped.
2. Inside dcmotorVoltage(), the raw terminal voltage input u4[3] was added after the Vmax clamp, bypassing the supply voltage limit.

Solution

1. In dcmotorVoltage(), add raw terminal voltage u4[3] and apply the Vmax clamp to the resulting total terminal voltage.
2. In mjGAIN_DCMOTOR, invoke dcmotorVoltage() whenever ctrlnum > 0 so that ctrl[uadr] is consistently clamped by Vmax.
3. Added unit test TEST_F(DCMotorTest, VmaxVoltageInput) to verify Vmax enforcement for voltage-controlled DC motors.

[Adityakk9031]

@yuvaltassa have a look