Refactor(tool): Enhance ToolCallbackAutoRegistrar for robustness and clarity

This commit refactors `ToolCallbackAutoRegistrar` to improve its
reliability, clarity, and initialization process.

Key changes include:
- Consolidated tool callback registration logic by integrating
  responsibilities from the (now removed) `ToolAnnotatedBeanProcessor`.
  This simplifies the auto-configuration flow for @tool annotated methods.
- Deferred tool scanning and `MethodToolCallbackProvider` registration
  until `ApplicationReadyEvent`. This ensures that the operation occurs
  only after the application context and all relevant beans are fully
  initialized, enhancing stability.

Signed-off-by: jitokim <[email protected]>

Author: jitokim