feat(BA-1428): Refactor event dispatcher and handlers directory structure

[HyeockJinKim]

resolves #4490 (BA-1428)

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[Copilot]

Pull Request Overview

This PR refactors the event dispatcher and handler structure, extracting scheduling logic into a dedicated handler, updating import paths to match the new directory layout, and adjusting the scheduler dispatcher initialization.

• Introduce ScheduleEventHandler and move scheduling event subscriptions out of SchedulerDispatcher
• Update SchedulerDispatcher to remove direct event consumption and adjust method signatures
• Update relative import paths in event_dispatcher/handlers/* and adjust context fixtures for tests

Reviewed Changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/manager/conftest.py	Add event_dispatcher_test_ctx fixture and import EventDispatcher
tests/manager/api/*.py	Replace usages of the old event_dispatcher_ctx fixture with the new one
src/ai/backend/manager/server.py	Pass scheduler_dispatcher into dispatcher args; refactor sched_dispatcher_ctx
src/ai/backend/manager/scheduler/dispatcher.py	Remove event consumption; change handler signatures to accept event_name
src/ai/backend/manager/event_dispatcher/handlers/*.py	Update relative imports from two- to three-dot paths after directory move
src/ai/backend/manager/event_dispatcher/dispatch.py	Wire up ScheduleEventHandler in Dispatchers; consolidate bgtask logic
src/ai/backend/manager/event_dispatcher/handlers/schedule.py	New file: implement handlers that delegate to SchedulerDispatcher
src/ai/backend/manager/config/unified.py	Update example values for IdleCheckerConfig
src/ai/backend/manager/api/context.py	Add scheduler_dispatcher attribute to RootContext

Comments suppressed due to low confidence (1)

src/ai/backend/manager/event_dispatcher/handlers/schedule.py:61

• The update_session_status method now requires an event_name argument; pass ev.event_name() to this call (e.g., await self._scheduler_dispatcher.update_session_status(ev.event_name())).

await self._scheduler_dispatcher.update_session_status()

[Copilot]

[nitpick] Import path for BraceStyleAdapter is inconsistent with other handlers; consider importing from ai.backend.logging to match existing usage.

Suggested change

	from ai.backend.logging.utils import BraceStyleAdapter
	from ai.backend.logging import BraceStyleAdapter