feat(backend): implement ClearTask and MarkTaskSuccess actions with scope, cache-invalidation, and comments. - #13996

Description of your changes:
This PR implements manual, per-task lifecycle controls for in-flight or failed pipeline runs. It adds backend support for scoped Clear (re-run task) and Mark Success (manually override task outcome) actions in coordination with the underlying Argo Workflow engine.

Key Additions:

1. API Schema (run.proto):
   • Added TaskScope enum (TASK_ONLY, DOWNSTREAM, UPSTREAM, UPSTREAM_DOWNSTREAM).
   • Defined ClearTaskRequest (includes invalidate_cache option) and MarkTaskSuccessRequest (includes mandatory comment field).
   • Added corresponding POST endpoints to RunService (:clear, :marksuccess).
2. Server Handlers (run_server.go):
   • Implemented server-side endpoints checking retry permission for clearing tasks and update permission for marking success.
   • Validates that comments for manual overrides are non-empty and non-whitespace.
   • Added Prometheus counter metrics tracking request rates.
3. DAG Resolution & mutations (resource_manager.go):
   • Formulates retries using Argo's native FormulateRetryWorkflow utility, safely preserving output and boundary IDs.
   • Restores unaffected/parent node statuses when executing scoped retries.
   • For Clear: if invalidateCache is enabled, appends the --cache_disabled argument to the task template definition in workflow.Spec so KFP driver bypasses the MLMD execution cache on retry.
   • For Mark Success: updates the target node phases to Succeeded and records the reason/comment in the node's status Message field for an audit trail.
   • Terminates running/pending pods associated with affected tasks to prevent races, resets workflow terminal flags, and notifies dispatcher plugins.
4. Verification:
   • Added unit tests in run_server_test.go checking validation constraints, cache-invalidation templating, phase updates, and audit comments.

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fixes #13883