Avoid rewriting cached aggregation keys during EVM setup

[Kuhai9801]

Summary

• reuse existing aggregation STARK and Halo2 proving keys when cargo openvm setup --evm is run without --force-agg-keygen
• only write aggregation key files after the command actually regenerates them
• keep verifier ASM and Solidity verifier generation working from the reused SDK keys

Changes

• Split the cached-key and keygen paths in crates/cli/src/commands/setup.rs.
• On the cached-key path, load the existing aggregation and Halo2 proving keys into the SDK and deserialize the existing aggregation verifying key as a validity check.
• On the keygen path, generate the aggregation keys and write the STARK proving key, STARK verifying key, and Halo2 proving key immediately after generation.
• Leave root verifier ASM and EVM verifier contract generation on the shared path.

Verification

• git diff --check
• cargo check -p cargo-openvm

Fixes #2356