feat(registry): Allow Cloud Engines to have blank replica_version_id (in their SubnetRecord).

[daniel-wong-dfinity-org-twin]

In order for that to be allowed, the following conditions must be met:

1. Primarily, StandardEngineReplicaVersionRecord exists
2. This feature must be enabled via flag.

As per our standard way of doing this, this is currently DISABLED.

The plan is to enable it once all subnets (or maybe, just Cloud Engines) understand how to handle a blank replica_version_id field.

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[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to 88e8390.

Security Overview

• 🔎 Scanned files: 2 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/f975f1a5-0a47-4a3b-b7f0-a55609e90390?scanId=d796a176-1640-4f4b-9ee4-db02726b0446&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Enhancement	► rs/registry/canister/src/flags.rs     Add IS_BLANK_REPLICA_VERSION_ID_FOR_CLOUD_ENGINES_ENABLED flag
Enhancement	► rs/registry/canister/src/invariants/replica_version.rs     Import new flag and adjust logic to handle blank replica_version_id for CloudEngines; add detailed comments and tests scaffolding

New code is not yet active in release builds. Will changelog when this feature is enabled.

[Copilot]

Pull request overview

Updates registry invariants to optionally allow CloudEngine subnets to have an empty replica_version_id in their SubnetRecord, gated behind a new (currently disabled) feature flag and the presence of a StandardEngineReplicaVersionRecord.

Changes:

• Add a new registry-canister flag for enabling blank replica_version_id specifically for CloudEngines.
• Update replica version invariants to ignore blank replica_version_id for CloudEngine subnets when the flag is enabled and a StandardEngineReplicaVersionRecord exists.
• Add tests covering the allowed/forbidden combinations (flag on/off, record present/absent, CloudEngine vs non-CloudEngine).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
rs/registry/canister/src/invariants/replica_version.rs	Adjusts version-in-use collection logic to permit blank CloudEngine replica_version_id under gated conditions; adds unit tests for the new behavior.
rs/registry/canister/src/flags.rs	Introduces a new feature flag and test-only temporary enable/disable helpers for blank CloudEngine replica_version_id.

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[pierugo-dfinity]

Looks great to me!