[rust-launcher] Verify no MPC node code reads MPC_IMAGE_HASH from env

[barakeinav1]

Context

Follow-up from PR #2326 (port node launcher to Rust). Tracked in #2598.

Problem

The Python launcher set MPC_IMAGE_HASH and MPC_LATEST_ALLOWED_HASH_FILE as container env vars. The Rust launcher injects equivalent data via the [tee] section in the TOML config file.

If any MPC node code still reads these from environment variables (e.g. std::env::var("MPC_IMAGE_HASH")), it will silently get empty/missing values.

Action

• Grep for MPC_IMAGE_HASH and MPC_LATEST_ALLOWED_HASH_FILE env reads in the MPC node codebase
• Verify the node reads these exclusively from the [tee] config section
• Remove any dead env var reading code if found