Describe the changes
Before:
Pending chunks cache used LRU eviction, allowing attackers to flood the cache with single-chunk entries and evict legitimate multi-chunk uploads.

After:
Cache evicts entries with the fewest chunks first, protecting multi-chunk uploads from single-chunk flooding attacks.

Changes

New Module: pending_chunks.rs

• Added PriorityPendingChunks struct with priority-based eviction
• Eviction selects the entry with the minimum chunk count instead of LRU
• Maintains per-entry LRU cache for individual chunks within each data_root

mempool_service.rs

• Replaced LruCache<DataRoot, LruCache<...>> with PriorityPendingChunks
• Simplified put_chunk() method signature (removed preheader_chunks_per_item parameter)
• Updated create_state() and test mock to use new type

chunks.rs

• Updated put_chunk() call site

Related Issue(s)
Please link to the issue(s) that will be closed with this PR.

Checklist

• Tests have been added/updated for the changes.
• Documentation has been updated for the changes (if applicable).
• The code follows Rust's style guidelines.

Additional Context
Add any other context about the pull request here.