trident: promote rollback to stable v1; adjust rollback return type to detect no-servicing - #729

…type with update/install

Two related API changes to tridentd's manual-rollback gRPC surface:

1. Promotes Rollback, RollbackStage, and RollbackFinalize from
   trident.v1preview.RollbackService to the stable trident.v1.RollbackService
   (proto/trident/v1/rollback_service.proto, new file). This gives rollback
   the same stable, caller-handles-reboot gRPC contract that
   UpdateStage/UpdateFinalize already have, instead of CLI-shelling.
   CheckRollback, GetRollbackChain, and GetRollbackTarget remain preview-only
   (see proto/trident/v1preview/rollback_service.proto).

2. Aligns manual rollback's no-op reporting with update()/install():
   - engine::manual_rollback::execute_rollback() and Trident::rollback() now
     return (ExitKind, ServicingType) instead of a bare ExitKind. A no-op
     (empty rollback chain, or host not in a rollback-eligible servicing
     state) now reports (ExitKind::Done, ServicingType::NoActiveServicing)
     rather than a bare ExitKind::Done indistinguishable from a real
     rollback at the gRPC layer.
   - The three gRPC handlers in services/rollback.rs now populate
     ServicingResponse.servicing_kind from that value, the same way
     update.rs/install.rs already do, instead of hardcoding None. This lets
     callers detect a no-op rollback from the response itself rather than
     needing a separate precondition query.
   - CheckRollback (which existed to answer that same "is a rollback
     available" question via a separate round-trip) is demoted back to
     trident.v1preview accordingly - its one caller no longer needs it.
     try_acquire_read_lock/reading_request/trident_error_to_status (and
     their Code/ErrorKind/OwnedRwLockReadGuard imports) are re-gated behind
     the grpc-preview feature, since check_rollback was their last
     non-preview caller.
   - main.rs's CLI `trident rollback` command updated for the new return
     shape (matches Install/Update/Commit's existing pattern).

Split out from user/bfjelds/acl-agent-rollback-grpc for isolated API
review: this covers only the tridentd-side proto/engine/server surface,
not trident-acl-agent's consumption of it.

Verified: cargo test -p trident --lib manual_rollback (22 passed), cargo
clippy -p trident --tests -- -D warnings (clean, default features), cargo
clippy -p trident --tests --all-features -- -D warnings (clean, confirms
grpc-preview-off build has no dead code from the CheckRollback demotion),
cargo fmt --check (clean).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 8c06585d-a82d-475f-a802-83fdfa012d86