Ensure OPP CDT models build before contracts

[huangminghuang]

Summary

This PR makes the contracts_project-build ExternalProject step depend directly on opp_cdt_models.protos.

Why this is necessary

sysio.opreg and other system contracts include the generated CDT protobuf headers from build/<config>/libraries/opp/generated-cdt. The parent build already created an opp_cdt_models.protos target for those headers and attached it to the outer contracts_project target, but contracts_project is an ExternalProject.

With Ninja, depending on the outer ExternalProject target is not enough to order the actual contracts_project-build step. Ninja could schedule the contract sub-build before regenerating stale CDT protobuf headers. That happened locally: attestations.proto contained ACTION_TYPE_WITHDRAW, actor, wire_account, and SlashOperator, but the generated CDT header still had the older ACTION_TYPE_WITHDRAW_REMIT/op_address shape. As a result, sysio.opreg failed to compile even though the proto source was correct.

By adding the dependency to the generated ExternalProject build target itself, contracts_project-build now waits for opp_cdt_models.protos before compiling contracts.

Verification

• Confirmed Ninja dry-run order changed to generate zpp protobuf headers before contracts_project.
• Confirmed build/debug/libraries/opp/generated-cdt/sysio/opp/attestations/attestations.pb.hpp regenerated with the current OperatorAction fields.
• Ran:

cmake -S . -B build/debug -DBUILD_SYSTEM_CONTRACTS=ON
cmake --build build/debug --parallel "$(nproc)"

The full build completed successfully.