feat: reserve-expanding sync eligibility for redistribution game (SWIP-44)

[aloknerurkar]

Checklist

• I have read the coding guide.
• My change requires a documentation update, and I have done it.
• I have added tests to cover my changes.
• I have filled out the description and linked the related issues.

Description

Implements SWIP-44: Reserve-Expanding Sync Eligibility.

Currently, Bee excludes a node from participating in the redistribution game whenever any historical pullsync is active (SyncRate() > 0). When a node is expanding its reserve into a lower bin ($d-1$) in preparation for a storage radius decrease, this sync can take hours, causing the node to miss rounds despite maintaining a complete and valid reserve at the current storage depth $d$.

Because the reserve digest $H(R_N(d))$ is computed strictly over PO bins $p \geq d$, chunks arriving in bins $p < d$ are disjoint from the reserve and do not influence the digest. This PR amends the eligibility check to allow participation during sub-depth sync:

1. pkg/puller:
   • Added bin-level active historical sync worker tracking (activeHistSyncs [swarm.MaxBins]int, activeHistSyncsMu sync.RWMutex).
   • Exposed IsBinSyncing(bin uint8) bool and depth-aware IsReserveSynced(depth uint8) bool.
   • Updated mock syncer and added TestIsReserveSynced.
2. pkg/storageincentives:
   • Updated Agent constructor and sampling handler to evaluate reserveSyncedFunc(committedDepth) instead of blanket global sync state.
   • Added unit tests covering game participation during sub-depth sync vs. reserve-depth sync.
3. pkg/node:
   • Wired pullerService.IsReserveSynced(depth) into storageincentives.New.

Open API Spec Version Changes (if applicable)
N/A

Motivation and Context (Optional)
Addresses SWIP-44 (ethersphere/SWIPs#92).

Related Issue (Optional)

• Addresses SWIP-44: eligibility for sampling and game participation during reserve-expanding pull-sync  SWIPs#92

Screenshots (if appropriate):
N/A

AI Disclosure

• This PR contains code that has been generated by an LLM.
• I have reviewed the AI generated code thoroughly.
• I possess the technical expertise to responsibly review the code generated in this PR.