[mcast] Lifecycle + API changes for Omicron impl

[zeeshanlakhani]

This change strengthens the multicast implementation with always-allocated group IDs, better API validation, and comprehensive test improvements for Omicron integration.

This update no longer generates multicast group IDs optionally. They are always allocated during group creation, following how multicast groups are configured in the Omicron CP

In Omicron, multicast groups are created first, without members, and then members are added as instances are configured for a multicast group.

Replication configuration is only written to tables when members are added, but IDs are always generated for the 1:1 mapping between underlay and external (overlay) associated groups.

Includes:

• Core ID Management Changes:

  • Remove Option - IDs are always allocated during
    group creation
  • Establish 1:1 mapping between underlay and external (overlay) groups
  • External groups now use IDs from corresponding NAT target (Omicron
    keeps the true relational mapping)

• API Changes and Validation:

  • Remove sources field from internal group
    APIs (MulticastGroupCreateEntry, MulticastGroupUpdateEntry)
  • New response types for External/Underlay: MulticastUnderlayGroupResponse and MulticastExternalGroupResponse, and unified for lists MulticastGroupResponse
  • Internal groups cannot have sources or NAT targets - cleaner
    separation of concerns
  • External groups retain sources for proper SSM (Source-Specific
    Multicast) validation
  • Now fail outright on reset if cleanup is not used properly, which
    helps on the Omicron side
  • Renaming API boundary structs to be consistent.
  • Integrate all the new types with the API trait that went in upstream
  • A new AdminScopedIpv6 Type to make the calls into dpd properly typed for internal underlay groups

• Rollback & Error Handling:

  • The addition of a rollback module (and trait) for a more
    functional approach to rollback on creation or updates involving
    tables, ports, etc
  • Improved error propagation in test cleanup to catch resource leaks early
  • Better validation of group ID relationships to match tables and
    allocation states

• Test Infrastructure Improvements:

  • Enhanced cleanup_test_group() to fail explicitly on deletion errors
    (prevents test pollution), and ensures proper 1:1 deletion mapping
  • New tests for rollback, empty members upon multicast group creation/update

• Replication Management:

  • Configure replication only when groups have members (change made
    expecting empty groups in Omicron CP initially)
  • Reconfigure replication tables when transitioning between empty/populated groups

Key aspects this commit covers:

1. ID Management to match expectations in Omicron's multicast impl
2. Validation: Enhanced API validation, group ID relationship checks,
   SSM validation
3. Rollback: Reset operations now fail explicitly, better error propagation
4. Testing: Comprehensive test improvements, better error handling,
   standardized cleanup

[FelixMcFelix]

Thanks, Zeeshan. Full disclosure that I haven't yet looked at the integration tests. I think the new rollback machinery is pretty neat -- obviously it's geared toward just multicast, but I think the model of maintaining a snapshot of the old target state and moving back to it is pretty useful.

I think overall I'm a bit confused by the mention of External forwarding groups being used with instances/guests, but most things here are nits.

[zeeshanlakhani]

@FelixMcFelix Sorry for the additional changes, as 2daa552 went in after the review. With it changing all the type handling, I went ahead and just made the API more consistent (and properly restrictive) across the board.