[BUG] Complex vjps are wrong for cos, sqrt, and rsqrt (missing conjugate) #3780

Describe the bug

mx.cos, mx.sqrt, and mx.rsqrt return the wrong gradient for complex inputs. cos's vjp delegates to its jvp, and sqrt/rsqrt form the derivative without conjugating it, so all three return f'(z) * cotangent instead of conj(f'(z)) * cotangent (the convention used by mx.exp/mx.log).

To Reproduce

import mlx.core as mx
z = mx.array(0.7 + 0.5j, mx.complex64)
g = mx.array(1.0 + 1.0j, mx.complex64)
print(mx.vjp(mx.cos, [z], [g])[1][0])
print(g * mx.conj(-mx.sin(z)))   # expected

Expected behavior

vjp == cotangent * conj(f'(z)), consistent with the other unary ops and finite differences.

Desktop

• OS: macOS
• Version: main (0.32.0.dev)