feat: Do not route canister messages from a cooling down subnet

[mraszyk]

Messages (requests and responses) in canister output queues are no longer routed into streams while the source subnet is cooling down. They are retained in their output queues until the subnet stops cooling down, rather than being rejected or dropped. This includes the loopback stream, i.e. a cooling down subnet does not route its canisters' messages to its loopback stream either.

The check is merged into the existing destination side check, so a queue is skipped (and counted in mr_cooling_down_skipped_queues) exactly once per round, whether it is the source subnet, the destination subnet, or both that are cooling down. As before, messages from the subnet's own output queues are exempt.

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to e60f5e5.

Security Overview

• 🔎 Scanned files: 3 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/f5e9d2bb-29a4-4b64-902a-391d65fd8b2d?scanId=a48f8430-02b9-4555-b519-8cd0440c8300&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► rs/messaging/src/routing/stream_builder.rs     Change metric description for cooling_down_skipped_queues to reflect either subnet cooling down, not just destination subnet; add own_subnet_is_cooling_down flag. ► rs/messaging/src/routing/stream_builder.rs     Use new condition to skip routing when either source or destination subnet is cooling down.
Bug Fix	► rs/messaging/src/routing/stream_builder.rs     Adjust logic to retain messages in output queues when either the source or destination subnet is cooling down, and only route once cooling is cleared.
Enhancement	► rs/messaging/src/routing/stream_builder/tests.rs     Update test fixtures and messages to reflect LOCAL_SUBNET cooling down behavior and new fixture naming. ► rs/messaging/src/routing/stream_builder/tests.rs     Add/adjust tests for retaining messages from cooling down subnets and subsequent routing when cooling down ends.
Refactor	► rs/replicated_state/src/metadata_state.rs     Improve documentation of cooling_down behavior, clarifying that cooling down retains messages in output queues and affects routing into streams.