[ECS-Plugin] Implement ECS_CANARY_CLEAN stage

[armistcxy]

What this PR does: Implement ECS_CANARY_CLEAN stage: Delete the canary taskset that previously deployed in ECS_CANARY_ROLLOUT stage

sequenceDiagram
        participant Plugin as ECSCanaryCleanStage
        participant Store as Metadata Store
        participant Provider as ECS Client
        participant ECS as AWS ECS

        Plugin->>Store: GetDeploymentPluginMetadata(canary-task-set)

        alt Metadata fetch error
            Plugin->>Plugin: Log error
            Plugin-->>Plugin: StageStatusFailure
        else Not found (no canary stage ran)
            Plugin->>Plugin: Log "nothing to clean up"
            Plugin-->>Plugin: StageStatusSuccess
        else Task set data found
            Plugin->>Plugin: Unmarshal TaskSet JSON
            alt Unmarshal error
                Plugin->>Plugin: Log error
                Plugin-->>Plugin: StageStatusFailure
            else Unmarshal success
                Plugin->>Provider: Create ECS client
                alt Client creation error
                    Plugin->>Plugin: Log error
                    Plugin-->>Plugin: StageStatusFailure
                else Client ready
                    Plugin->>ECS: DeleteTaskSet
                    alt TaskSetNotFoundException (already deleted — idempotent)
                        Plugin->>Plugin: Log "already deleted, skipping"
                        Plugin-->>Plugin: StageStatusSuccess
                    else Delete error
                        Plugin->>Plugin: Log error
                        Plugin-->>Plugin: StageStatusFailure
                    else Delete success
                        Plugin-->>Plugin: StageStatusSuccess
                    end
                end
            end
        end

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Why we need it: Clean up the canary taskset

Which issue(s) this PR fixes: Part of #6443

Fixes #

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[khanhtc1202]

LGTM