💥 Standalone Activities for Ruby

[GregoryTravis]

Add standalone activity API (Client#start_activity + friends)

Introduces support for standalone activities — activities that execute independently of any workflow.

💥 Breaking Changes:

• Activity::Info#workflow_id, #workflow_run_id, #workflow_type are now nullable (String?); they return nil when the activity is standalone.
• Activity::Info#workflow_namespace is now marked @deprecated in favor of the new #namespace accessor. Both fields always carry the same value regardless of standalone-vs-workflow context; the rename matches the cross-SDK convention.

New public API surface:

• Client#start_activity / Client#execute_activity — start a standalone activity execution by name, class, instance, or Activity::Definition::Info; execute_activity is start_activity + handle.result as a shortcut
• Client#activity_handle(activity_id, activity_run_id:, result_hint:) — get a handle to an existing standalone activity (e.g. one started by a different process)
• Client#list_activities(query) / Client#count_activities(query) — visibility queries over standalone activities; list_activities returns an Enumerator<ActivityExecution>
• Client::ActivityHandle — handle returned by start_activity / activity_handle; provides #result(result_hint:, rpc_options:), #describe, #cancel(reason), #terminate(reason)
• Client::ActivityExecution — lightweight metadata used in list_activities results
• Client::ActivityExecution::Description — rich descriptor returned by ActivityHandle#describe; subclass of ActivityExecution. Exposes ~25 fields from the proto (status, attempt, retry_policy, last_failure, last_heartbeat_time, priority, canceled_reason, static_summary/static_details, etc.)
• Client::ActivityExecutionCount + ::AggregationGroup — result of count_activities(), with group-by support
• Client::ActivityExecutionStatus — enum module wrapping the proto's status values
• ActivityIDReusePolicy + ActivityIDConflictPolicy — enum modules in lib/temporalio/common_enums.rb mirroring the workflow-side policies
• Error::ActivityAlreadyStartedError (in error/failure.rb) — raised by start_activity when the server rejects a duplicate ID
• Error::ActivityFailedError (in error.rb) — raised by handle.result when the activity terminates in failure; cause carries the underlying failure (ApplicationError, CanceledError, TimeoutError, TerminatedError)

New interceptor API:

• Client::Interceptor::StartActivityInput, DescribeActivityInput, CancelActivityInput, TerminateActivityInput, ListActivitiesInput, CountActivitiesInput, FetchActivityOutcomeInput — seven new Data.define input types
• Client::Interceptor::Outbound extended with seven matching pass-through methods (start_activity, describe_activity, cancel_activity, terminate_activity, list_activities, count_activities, fetch_activity_outcome)
• Chain ordering convention is first-added-is-outermost.

Activity::Info additions:

• #activity_run_id — run-scoped id assigned by the server to each standalone activity execution (nil for workflow-dispatched)
• #namespace — canonical namespace accessor (preferred over the deprecated #workflow_namespace)
• #in_workflow? — predicate distinguishing workflow-dispatched activities from standalone ones

Client#async_activity_handle extension (no new method):

• Now accepts an ActivityIDReference constructed via ActivityIDReference.for_standalone(activity_id:, activity_run_id:).

ActivityIDReference additions:

• ActivityIDReference.for_standalone(activity_id:, activity_run_id: nil) — class factory for the new standalone shape.
• #standalone? — predicate for branching in async-completion routing

CI / dev server:

• Enable standalone-activity server feature flags in the Temporal CLI dev server used by unit tests: frontend.activityAPIsEnabled, activity.enableStandalone, history.enableChasm, history.enableTransitionHistory (added to test/test.rb).
• Bumped sdk-core submodule to a22517e4 (Rust crate 0.4.0) to pick up the worker-side coresdk.activity_task.Start.run_id proto field used to plumb activity_run_id into Activity::Info.

Tests:

• test/client_activity_test.rb (28 tests) — integration tests covering the full lifecycle: start, execute, by-name, already-started, timeouts, failure-throws-ActivityFailedError, describe (running / terminated / canceled / retried / raw-info parity), state-transition-count, terminate-result-throws, polling-correctness (with timing assertions across one server long-poll deadline boundary), list, count, get-handle-with-nil-run-id, cancel-transitions-to-CANCELED, retry-policy / priority / id-reuse / id-conflict-policy round trips
• test/client_activity_async_completion_test.rb (6 tests) — async completion via AsyncActivityHandle against a standalone ActivityIDReference: complete by activity_id, complete with run_id, heartbeat, fail, heartbeat+fail, report_cancellation
• test/client_activity_hints_test.rb (6 tests) — definition hints used for client-side arg encode and worker-side decode, definition result_hint used for worker-side encode and client-side decode, call-site overrides on start_activity, by-name activities produce nil hints, handle.result(result_hint:) overrides, activity_handle(id, result_hint:) constructor-time hint, AsyncActivityHandle#complete(result, result_hint:) propagates the hint
• test/client_activity_interceptor_chain_test.rb (5 tests) — multi-interceptor ordering for activity client calls; verifies first-added-is-outermost
• test/client_workflow_interceptor_chain_test.rb (2 tests) — mirror of the above for the workflow client chain
• test/activity_info_standalone_test.rb (2 tests) — Activity::Info standalone-vs-workflow fields asserted from inside the activity body
• test/client/activity_id_reference_test.rb (5 tests) — factory shapes, for_standalone correctness, both forms coexist independently

Documentation:

• README.md — added "Standalone Activities" subsection under "Activities" with five runnable code examples (start, execute, get-handle + describe/result/cancel/terminate, list/count, in-activity-body context introspection)