[rust-launcher] Confirm RUST_LOG / RUST_BACKTRACE passthrough to MPC node

[barakeinav1]

Context

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

Problem

The Python launcher passed RUST_LOG, RUST_BACKTRACE, and NEAR_BOOT_NODES as --env flags to the MPC container. The Rust launcher passes zero env vars to the container (except DSTACK_ENDPOINT in TEE mode via compose template).

These must now be set either:

• In [mpc_node_config] TOML and read by the node from the config file, or
• Via the compose template as environment variables

Action

1. Verify how RUST_LOG and RUST_BACKTRACE reach the MPC node process
2. If they don't, decide on approach (compose template env section vs config file)
3. Update documentation for operators migrating from Python launcher