fix(coprocessor): decompress all ciphertexts per operation

[Eikix]

Closes zama-ai/fhevm-internal#1086

Summary

Rebuild this fix from main with a single focused change:
force ciphertext dependencies through a compressed-at-hop path, and decompress only at operation execution.

This aligns with the updated consensus model:
compress/decompress and rerand are both representation-changing transforms and must be on the same deterministic path.

What changed

• coprocessor/fhevm-engine/scheduler/src/dfg/scheduler.rs

  • remove transaction-wide pre-decompression path
  • when filling missing tx inputs from prior results, always propagate as DFGTxInput::Compressed
  • in try_execute_node, accept compressed task inputs and decompress right before rerand/compute
  • in run_computation, always compress operation outputs (not only allowed)

• coprocessor/fhevm-engine/scheduler/src/dfg.rs

  • propagate outputs to dependents as compressed task inputs
  • store task results as (ct_type, compressed_ct) payload

• coprocessor/fhevm-engine/scheduler/src/dfg/types.rs

  • simplify TaskResult to compressed payload only:
    • ct_type
    • compressed_ct
    • is_allowed
    • transaction_id

Deterministic path now

For ciphertext dependencies:
compressed -> decompress -> rerand -> compute -> compress

Validation

cd coprocessor/fhevm-engine
cargo check -p scheduler
SQLX_OFFLINE=true cargo test -p scheduler --lib

[mergify]

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