fix: reject overflowing multisig signer indexes in gRPC compat decode

[Ekleog-Polygon]

Summary

• reject u32::MAX multisig signer indexes in v1 gRPC compat decode with the typed Multisig ECDSA signer index overflow error
• keep the decode path from panicking in debug and test builds by using checked arithmetic when sizing the signer vector
• add a regression test that asserts the overflow case returns the exact invalid-data error

Testing

• cargo test -p agglayer-interop-grpc-types

Fixes #214