fix(app): prune block headers and improve sync value handling

[mpoke]

Summary

• Return decoding errors instead of panicking in decode_value
• Handle decode failures gracefully in on_process_synced_value by sending None reply
• Treat invalid synced payloads as errors (since 2/3+ validators accepted them, failure indicates serious issues)
• Ensure synced values are stored as undecided so on_decided() can retrieve block data
• Add caching for earliest certificate and value heights to avoid repeated DB lookups
• Fix unbounded growth of DECIDED_BLOCK_HEADERS_TABLE by pruning it alongside certificates
• Expand store tests to verify block header pruning

Changes

Error handling improvements

• decode_value now returns Result instead of unwrapping
• replay_heights_to_engine propagates decode errors with context
• on_process_synced_value sends None reply on decode failure instead of panicking

Sync value handling

• Invalid synced payloads now return an error (indicates state divergence or Byzantine behavior)
• Synced values are stored as undecided so block data is available when decided

Performance

• Cache earliest_certificate_height and earliest_value_height in State
• Cache is lazily loaded on first access and updated after pruning

Bug fix

• DECIDED_BLOCK_HEADERS_TABLE is now pruned alongside certificates (was growing unbounded)

[jmalicevic]

I wonder whether the panic here is an attack vector on a node. Namely, if you send me an invalid proposal, instead of making it invalid, I crash here without holding you accountable.

[mpoke]

Malachite verifies the certificate before passing the proposal to the application. This means that a peer can only send an invalid proposal if it can create a valid certificate, which is only possible if more than 1/3 of the voting power is malicious.

[jmalicevic]

Ok so indeed this is an invalid scenario, I checked malachite's code to make sure that the certificates are indeed validated before the value is validated (in a previous version, those validations were happening in parallel). So yes, this is a serious problem which should be cause a crash,