serviceability: add owner field to UpdateMulticastGroup instruction

[juan-malbeclabs]

Summary of Changes

• Adds an optional owner field to MulticastGroupUpdateArgs, allowing foundation members to reassign ownership of a multicast group after creation
• Adds --owner CLI flag to doublezero multicast group update, accepting a pubkey or me (resolves to the active keypair)
• New field is appended after use_onchain_allocation in the Borsh encoding, so older serialized instructions continue to deserialize correctly via BorshDeserializeIncremental

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	3	+20 / -4	+16
Scaffolding	1	+1 / -0	+1
Tests	4	+200 / -30	+170

Small core change, most of the diff is new and refactored unit tests.

Key files (click to expand)

• smartcontract/cli/src/multicastgroup/update.rs — adds --owner arg with validate_pubkey (accepts me), resolves me to the payer pubkey in execute; adds three new unit tests covering no owner, explicit pubkey, and me resolution
• smartcontract/programs/doublezero-serviceability/src/processors/multicastgroup/update.rs — adds owner: Option<Pubkey> to MulticastGroupUpdateArgs and applies the update in the processor
• smartcontract/sdk/rs/src/commands/multicastgroup/update.rs — adds owner: Option<Pubkey> to UpdateMulticastGroupCommand and threads it through to the instruction args

Testing Verification

• All existing multicast group integration tests pass (test_multicastgroup, test_multicastgroup_deactivate_*, test_delete_multicastgroup_*, test_update_multicastgroup_*)
• test_multicastgroup extended to set owner: Some(new_owner) in the update step and assert the new owner is persisted on-chain
• Three new CLI unit tests: test_cli_multicastgroup_update_with_explicit_owner, test_cli_multicastgroup_update_owner_me, and test_cli_multicastgroup_update_owner_parsing