refactor: generalize hybrid attention checkpoint management

[shihaobai]

Summary

Extract the existing linear/full checkpoint flow into reusable hybrid interfaces, without including the Gemma/sliding-window implementation.

• Add HybridAttentionReqManager for request-state initialization, big/small-page snapshots, restoration and model-specific MTP state updates. Conv/SSM access stays inside the linear implementation.
• Move checkpoint storage to common/state_cache_manager/: shared slot allocator and layer storage, with the linear config and buffers together in linear_att.py.
• Generalize request fields, paged radix-cache types, multi-level cache routing and PD state-page dispatch across the repository.
• Preserve checkpoint creation timing, size-first CPU layouts, page matching/eviction boundaries, CPU/disk packing and existing --linear_att_* options. Model-specific PD packing/restoration and payload layouts remain unchanged. Rename the PD page kind from linear_att_state to att_state and document its boundary-state semantics; P and D must be updated together because the kind is part of the task identity.