feat: ic0_subnet_self_node_count

[eichhorl]

Implement a new system API function allowing a canister to determine the size of its current subnet. This will be helpful for flexible outcalls, in which a canister can individually specify how many replicas should perform a request, and how many of them should produce a response. This only makes sense within the context of the own subnet size.

Specified here (draft): dfinity/developer-docs#254

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to 7bf17b3.

Security Overview

• 🔎 Scanned files: 10 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/f5e9d2bb-29a4-4b64-902a-391d65fd8b2d?scanId=9ce12d31-0b73-46c9-bd91-a524b1a01055&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► rs/embedders/src/wasm_utils/validation.rs     Add subnet_self_node_count system API variant in get_valid_system_apis_common
Enhancement	► rs/embedders/src/wasmtime_embedder/linker.rs     Wrap and expose ic0.subnet_self_node_count syscall in linker
Enhancement	► rs/embedders/src/wasmtime_embedder/system_api.rs     Implement ic0_subnet_self_node_count in SystemApiImpl
Enhancement	► rs/embedders/src/wasmtime_embedder/system_api_complexity.rs     Add SUBNET_SELF_NODE_COUNT complexity cost
Enhancement	► rs/embedders/tests/system_api.rs     Update is_supported to include SubnetSelfNodeCount, and test availability handling
Enhancement	► rs/embedders/tests/wasmtime_embedder.rs     Import ic0_subnet_self_node_count in wasm64 tests
Enhancement	► rs/execution_environment/src/query_handler/query_cache/tests.rs     Include SubnetSelfNodeCount in supported system APIs for query cache tests
Enhancement	► rs/execution_environment/tests/hypervisor.rs     Add WAT for ic0.subnet_self_node_count and related tests for node count behavior
Enhancement	► rs/interfaces/src/execution_environment.rs     Add SubnetSelfNodeCount to SystemApiCallId and SystemApi trait signature ic0_subnet_self_node_count
Enhancement	► rs/test_utilities/execution_environment/src/lib.rs     Add with_subnet_size builder to configure subnet size and related node IDs