feat(server): support push configs and gRPC list interop

[muscariello]

Summary

• normalize gRPC ListTasks page sizes so Go clients sending an unset value as zero still receive the default result window
• add opt-in push-config CRUD support to DefaultRequestHandler via with_push_config_store(...)
• add protobuf, task-store, handler, and gRPC end-to-end regressions for the interop paths

Testing

• cargo fmt --all --check
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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