statement-store: close light node test coverage gaps

[AndreiEres]

Summary

Tracking issue for the remaining scenarios in statement-store light node tests. See details in the testing strategy.

Tasks

P0

• Wire protocol: unit tests cover edge cases around truncated payloads and compact encoding boundaries, polkadot-sdk cross-compatibility, can be extended with fuzzing
• Submission: unit tests cover result mapping, Zombienet tests verify statements sent by the light node reach connected full nodes
• Reception: unit tests cover bloom filter construction, false-positive discard, deduplication, subscription notifications, and unsubscribe; Zombienet tests verify only subscribed statements are delivered without duplicates
• Peer connection: Zombienet tests verify connectivity is maintained or restored when full nodes drop

P1

• Long-session stability: Versi tests verify a light node remains stable while connected to real full nodes for hours
• Browser sanity check: Zombienet tests verify the basic flow works in a headless or real browser environment