apollo_propeller: fix UnitValidationError error messages to say 'unit' instead of 'shard'

[sirandreww-starkware]

Note

Low Risk
Low risk: string-only changes to error messages with no behavior or data-flow impact.

Overview
Updates UnitValidationError’s thiserror messages in types.rs to consistently use unit terminology (instead of shard) across self-send, duplicate, schedule/parent verification, signature, and Merkle proof failure cases.

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[reviewable-StarkWare]

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[sirandreww-starkware]

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• apollo_protobuf: fix PropellerUnit proto comments to say 'unit' instead of 'shard' #13540
• apollo_propeller: fix sharding_test comment to say 'units' instead of 'shards' #13536
• apollo_propeller: fix typo 'form' to 'from' in SignatureVerificationError message #13535
• apollo_propeller: rename test_duplicate_shard_rejected to test_duplicate_unit_rejected #13534
• apollo_propeller: fix remaining tree.rs comments to say 'unit' instead of 'shard' #13533
• apollo_propeller: fix get_peer_for_shard_index doc comment to say 'unit' #13532
• apollo_propeller: rename shard_count to unit_count in should_build and should_receive #13531
• apollo_propeller: fix PropellerScheduleManager doc comments to say 'units' #13530
• apollo_propeller: remove incorrect tree.rs module doc comment #13529
• apollo_propeller: rename shard_count/total_shards to unit_count/total_units #13528
• apollo_propeller: rename shards param to units in reconstruct_blocking #13527
• apollo_propeller: fix PostConstruction doc comment to say 'units' #13526
• apollo_propeller: rename DuplicateShard/ReceivedSelfPublishedShard variants #13525
• apollo_propeller: fix UnitValidationError error messages to say 'unit' instead of 'shard' #13524 👈 (View in Graphite)
• apollo_propeller: fix UnitValidationError doc comment to say 'unit verification' #13523
• apollo_propeller: fix SignatureVerificationError error message to say 'unit' #13522
• apollo_propeller: fix Event doc comments to say 'units' instead of 'shards' #13520
• apollo_propeller: rename received_shards param to received_units in reconstruct_data_shards #13509
• apollo_propeller: rename ShardSendFailed/ShardValidationFailed event variants #13508
• apollo_propeller: rename ShardSignatureVerificationError to SignatureVerificationError #13507
• apollo_propeller: rename ShardValidationError to UnitValidationError #13506
• apollo_propeller: rename ShardPublishError to UnitPublishError #13505
• apollo_propeller: rename shards_received metric to units_received #13504
• apollo_propeller: rename validate_shard to validate_unit in UnitValidator #13503
• apollo_propeller: rename local vars shards/shard_count to units/unit_count in handle_action #13502
• apollo_propeller: rename maybe_broadcast_my_shard to maybe_broadcast_my_unit #13501
• apollo_propeller: rename num_held_shards to num_held_units #13500
• apollo_propeller: rename did_broadcast_my_shard to did_broadcast_my_unit #13499
• apollo_propeller: rename received_shards to received_units in ReconstructionState #13498
• apollo_propeller: fix tree.rs comments to use 'unit' instead of 'shard' #13497
• apollo_propeller: fix unit_validator.rs comments to use 'unit' instead of 'shard' #13496
• apollo_propeller: fix message_processor.rs comments to use 'unit' instead of 'shard' #13495
• apollo_propeller: fix engine.rs comments, use 'unit' instead of 'shard' #13494
• apollo_propeller: replace local Stake type alias with StakingWeight from starknet_api #13037
• apollo_propeller: multiple shards per peer #12914
• apollo_propeller: changed CommitteeId to be a hash value #12913
• apollo_propeller: renamed Channel to CommitteeId #12911
• apollo_propeller: widen ShardIndex from u32 to u64 #12878
• apollo_propeller: e2e tests #12837 : 1 other dependent PR (#12838 )
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