test(e2e): clarify backend-specific task names

[elezar]

Summary

Reorganize the E2E mise task names so their backend intent is explicit. This is a task naming and wiring cleanup only; it does not add new test coverage.

Related Issue

N/A

Changes

• Route the default e2e task through the explicit Docker-backed suite name.
• Move Docker-backed Rust and Python task entry points under e2e:docker:* names.
• Rename the k3s-cluster GPU task entry point to e2e:k3s:gpu.
• Keep the old mise task names as hidden warning wrappers that shell out to the new task names.
• Factor the Docker-backed Rust runner into e2e/rust/e2e-rust.sh and keep e2e/rust/e2e-docker.sh as a script-level compatibility wrapper.
• Update workflow commands and TESTING.md to use the clearer task names.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)
• mise run --dry-run e2e:rust ::: e2e:gpu ::: e2e:docker:rust verified deprecated wrapper ordering

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

[elezar]

@drew does e2e:k3s:gpu feel like a reasonable name for the GPU E2E task that runs through the k3s-cluster gateway, or would you prefer a different name?

[elezar]

Note that the switch to the docker gateway was made in #1153.