test: return Result from clicking_without_mouse_tracking_is_a_typed_error

[cestercian]

What & why

clicking_without_mouse_tracking_is_a_typed_error was the one test in crates/termlens/tests/input.rs that did not return termlens::Result<()>. The other 25 do, and SKILL.md rule 9 asks tests to. The four unwrap()s (spawn, wait_until, send, wait_exit) become ? and the test ends with Ok(()). The unwrap_err() on the click stays, since that error is the thing under test.

Closes #544

Checklist

• Linked an issue (or explained above why none exists)
• Tests added/updated for the change
• cargo fmt --all and cargo clippy --workspace --all-targets --all-features are clean (clippy run on -p termlens --tests)
• All commits are signed off (git commit -s)
• No AI attribution trailers
• CHANGELOG.md updated under [Unreleased] (user-facing changes only): n/a, test-only
• Snapshot changes (if any) were reviewed with cargo insta review, not blind-accepted: n/a

cargo test -p termlens --test input clicking_without_mouse_tracking passes.