[chore] advance architecture refactor

[ldmonster]

Overview

Extract interfaces and named types to decouple ShellOperator's core components, replace an untyped task-type switch with a registry, and remove dead code. All changes are backward-compatible; no public API is broken.

What this PR does / why we need it

HookManager interface (pkg/hook/hook_manager.go)

• Defined a HookManager interface capturing the 10 methods used by ShellOperator.
• Changed ShellOperator.HookManager from *hook.Manager (concrete) to hook.HookManager (interface).
• Added var _ HookManager = (*Manager)(nil) compile-time assertion.
• Effect: ShellOperator no longer imports the concrete Manager; tests can inject a stub with zero real dependencies.

HookDiscovery interface (pkg/hook/hook_manager.go)

• Extracted HookDiscovery interface with a single Discover(workingDir string) ([]string, error) method.
• Added FileSystemHookDiscovery as the default implementation (wraps the existing utils_file.RecursiveGetExecutablePaths call).
• Manager.Init() now calls hm.hookDiscovery.Discover(...) instead of the utility function directly.
• ManagerConfig.HookDiscovery field added; nil defaults to FileSystemHookDiscovery{}.
• Effect: filesystem coupling removed from Manager.Init; tests can provide a deterministic stub without touching the real filesystem.

Named webhook event handler types (pkg/shell-operator/webhook_handlers.go, new file)

• Introduced TaskRunner type alias (func(ctx, task) TaskResult).
• Extracted AdmissionEventHandler struct with constructor NewAdmissionEventHandler(hm, runner, logger) and Handle method matching the WithAdmissionEventHandler signature.
• Extracted ConversionEventHandler struct with constructor NewConversionEventHandler(hm, runner, logger) and Handle method matching EventHandlerFn.
• Replaced the ~45-line inline closure in initValidatingWebhookManager with NewAdmissionEventHandler(...).Handle.
• Replaced the dead conversionEventHandler method (which also had a return res undefined-variable compile error from the merge) and the inline-style call in initConversionWebhookManager with NewConversionEventHandler(...).Handle.
• Effect: event-handling logic is now independently testable, named, and dependency-injected.

TaskHandlerRegistry (pkg/shell-operator/task_handler_registry.go, new file)

• Defined TaskHandlerFunc and TaskHandlerRegistry (map from task.TaskType to handler).
• NewTaskHandlerRegistry(), Register(taskType, handler), Handle(ctx, task) (TaskResult, bool).
• Added taskHandlerRegistry *TaskHandlerRegistry field to ShellOperator.
• Added taskHandler(ctx, task) TaskResult dispatcher method (thin wrapper over the registry).
• Added RegisterBuiltinTaskHandlers() method registering HookRun, EnableKubernetesBindings, and EnableScheduleBindings; called from assembleShellOperator in bootstrap.go.
• Extracted taskHandleEnableScheduleBindings from the switch as a proper method alongside the existing taskHandleHookRun and taskHandleEnableKubernetesBindings.
• Effect: adding a new task type no longer requires editing a central switch; addon-operator can call Register() to extend without forking.

Dead code removal

• CommonHook interface (pkg/hook/hook.go): was never referenced outside its own declaration — removed.
• FlagInfo.Define bool field (pkg/app/app.go): all callers always passed true; the if flag.Define { } branches were unconditional in practice — field and branches removed, flag registration is now always unconditional.
• conversionEventHandler method on ShellOperator: superseded by ConversionEventHandler — removed.
• v1 "k8s.io/apiextensions-apiserver/pkg/apis/apiextensions/v1" import in operator.go: became unused after the above removal — removed.