chore: support canister migration from/to deleted subnet in registry

[mraszyk]

This PR supports canister migration from/to deleted subnet in the registry by:

• adding a test that canister migration from a deleted (source) subnet succeeds (the implementation already supports this case so we just add a test to prevent a regression);
• skipping updating the deleted target subnet in the routing table (the canister migration is still successful since the outcome should be as if the migration completed first and the subnet was only deleted later) and adding a test covering this case.

[Copilot]

Pull request overview

This PR updates the registry canister’s canister-migration logic to tolerate migrations involving subnets that have been deleted from the registry/routing table, and adds regression tests to lock in the behavior.

Changes:

• Adjust routing-table mutation so migrating to a target subnet that is absent from the routing table does not leave any ranges assigned to that (deleted) target subnet.
• Relax payload validation to allow migrations to target subnets that no longer exist in the registry.
• Add/extend unit tests covering migration when the source subnet is “deleted” (unrouted gap) and when the target subnet does not exist.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
rs/registry/canister/src/mutations/routing_table.rs	Skips persisting assignments to a target subnet that has no existing ranges in the routing table.
rs/registry/canister/src/mutations/do_migrate_canisters.rs	Removes target-subnet existence validation and adds tests for deleted-source / deleted-target migration scenarios.

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[github-actions]

This pull request changes code owned by the Governance team. Therefore, make sure that
you have considered the following (for Governance-owned code):

1. Update unreleased_changelog.md (if there are behavior changes, even if they are
   non-breaking).

2. Are there BREAKING changes?

3. Is a data migration needed?

4. Security review?

How to Satisfy This Automatic Review

1. Go to the bottom of the pull request page.

2. Look for where it says this bot is requesting changes.

3. Click the three dots to the right.

4. Select "Dismiss review".

5. In the text entry box, respond to each of the numbered items in the previous
   section, declare one of the following:

• Done.

• $REASON_WHY_NO_NEED. E.g. for unreleased_changelog.md, "No
  canister behavior changes.", or for item 2, "Existing APIs
  behave as before.".

Brief Guide to "Externally Visible" Changes

"Externally visible behavior change" is very often due to some NEW canister API.

Changes to EXISTING APIs are more likely to be "breaking".

If these changes are breaking, make sure that clients know how to migrate, how to
maintain their continuity of operations.

If your changes are behind a feature flag, then, do NOT add entrie(s) to
unreleased_changelog.md in this PR! But rather, add entrie(s) later, in the PR
that enables these changes in production.

Reference(s)

For a more comprehensive checklist, see here.

GOVERNANCE_CHECKLIST_REMINDER_DEDUP

[mraszyk]

I believe this PR is not worth an entry in the changelog...

[zeropath-ai]

✅ No security or compliance issues detected. Reviewed everything up to 7e5ff24.

Security Overview

• 🔎 Scanned files: 2 changed file(s)
• 🔗 Scan Link: https://zeropath.com/app/repositories/f975f1a5-0a47-4a3b-b7f0-a55609e90390?scanId=3e4b2f95-d111-4a69-8a5c-627f965a917f&codeScanTypes=PrScan&tab=issues

Detected Code Changes

Change Type	Relevant files
Refactor	► rs/registry/canister/src/mutations/do_migrate_canisters.rs     Remove unnecessary validation for target subnet existence in do_migrate_canisters     Add test for migrating canisters when the source subnet is deleted     Add test for migrating canisters to a non-existent target subnet ► rs/registry/canister/src/mutations/routing_table.rs     Modify routing table assignment to handle non-existent target subnets
Enhancement	► rs/registry/canister/src/mutations/do_migrate_canisters.rs     Update test to include dedicated range for target subnet     Update test assertion for routing table split around migrated canister