[submit] SubmitError missing #[non_exhaustive] — future variants will be semver-breaking

[obchain]

Refs #44

File: crates/charon-executor/src/submit.rs

Problem

Pattern recurring since PR #27. All public error enums must carry #[non_exhaustive] so new variants (RateLimited, AuthFailure, etc) can be added without breaking downstream match exhaustiveness.

Fix

Add #[non_exhaustive] to SubmitError enum (introduced by sibling issue on this PR).

[obchain]

Addressed together with #195 — SubmitError carries #[non_exhaustive] from its initial definition.