Preserve table signature when constructing tables from checkpoint protos

[rootkiller6788]

Problem

server_from_proto.tables_from_proto reconstructs reverb.Table objects from PriorityTableCheckpoint protos but never passed the signature field to reverb.Table. PriorityTableCheckpoint.signature is a tensorflow.StructuredValue (field 9 in reverb/cc/checkpointing/checkpoint.proto) that the C++ Table::Checkpoint() writes and Table::InitializeFromCheckpoint() restores. The Python path dropped it, so any table rebuilt from a checkpoint proto lost its signature.

A signature-less table means TrajectoryDataset.from_table_signature and Client.sample(..., unpack_as_table_signature=True) can no longer reconstruct the nested data structure, and Table.info().signature returns None.

Fix

Decode config.signature with tensorflow.python.saved_model.nested_structure_coder.decode_proto (the same codec TableInfo.from_serialized_proto uses) and pass the result to reverb.Table(signature=...). When the field is unset, signature=None preserves the previous behavior.

Test

Added ServerFromProtoTest.test_table_from_proto_preserves_signature, which encodes a tf.TensorSpec signature into a PriorityTableCheckpoint, converts it via tables_from_proto, and asserts tables[0].info.signature equals the original signature.