Isolate known attribute names for type-based @SessionAttributes by handler type

[froggy0m0]

Problem

SessionAttributesHandler manages the session attributes that a specific handler declares through @SessionAttributes.

With type-based @SessionAttributes, the actual model attribute names are discovered at runtime.
SessionAttributesHandler therefore stores these known attribute names in the session
so they can be restored in distributed session scenarios.

However, all handler types currently store their known attribute names under the same SESSION_KNOWN_ATTRIBUTE key.
As a result, one handler can treat names recorded by another handler as its own.

Consequently, one handler can restore session attributes managed by another.
If that handler subsequently calls SessionStatus#setComplete(),
the cleanup that follows can also remove session attributes managed by the original handler.

Changes

Include the handler type name in the session key used to store and restore known attribute names.

This keeps the known attribute names for type-based @SessionAttributes isolated by handler type.
It does not change how the actual session attributes are stored or how name-based
@SessionAttributes declarations behave.

Tests

Add regression tests that verify the following:

• Session attributes managed by a different handler type are not restored.
• Cleanup for one handler type does not remove session attributes managed by another handler type.

---

I would appreciate your review of these changes. I can make changes if needed.