Proto serialization of `FrozenCircuit` doesn't serialize `tags`

[andbe91]

Describe the issue

When serializing a FrozenCircuit using Protos, it disregards the tags attribute. Note that the json one works fine.

Explain how to reproduce the bug or problem

import cirq
import cirq_google.serialization.circuit_serializer as circuit_serializer

circuit = cirq.FrozenCircuit(tags=("foo"))
circuit_serializer.CIRCUIT_SERIALIZER.serialize(circuit)

Tell us the version of Cirq where this happens

1.6.0.dev20250519212157