statement-store: Add malformed input unit tests

[AndreiEres]

Description

Part of #11534: covers tests for malformed input

Primitives crate:

• decode_checks_fields — extended with more duplicate/out-of-order field cases
• decode_rejects_malformed_bytes — new test: corrupted encoded bytes (empty input, invalid
  discriminants, truncated payloads, invalid proof variants, inflated field counts)
• sign_and_verify — extended with Ed25519 and ECDSA invalid signature cases

Client crate:

• submit_rejects_malformed_statements — new test: no proof, corrupted signatures (all 3 key types),
  wrong signer

Integration

No integration needed

[AndreiEres]

/cmd prdoc --audience node_dev --bump none