#20654 cmux-tui: clear the base clippy red in replace_unresponsive_owner

Summary

This fixes a base red on feat-cmux-next. cargo clippy -D warnings fails with "unnecessary qualification" at cmux-tui/crates/cmux-tui/tests/replace_unresponsive_owner.rs:92. The std::mem::size_of there was added with cx-jniyz (54fe01eb7e83e), and size_of is already in the prelude. This PR drops the path, a one-line change in a test file.

Verification

• cmux-ci run --class isolated --script scripts/ci/cmux-tui-rust-check.sh --arg clippy on this head (results in a comment below).
• Red: the same check on #20360's head 104f527ecbfd failed on this line (fleet step bf708a8ffee9f738757622cf).

Changelog

none

🤖 Generated with Claude Code

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Summary by cubic

Clears the clippy failure on feat-cmux-next by dropping the std::mem:: path from size_of in the replace_unresponsive_owner test.

cargo clippy -D warnings reports "unnecessary qualification" on this line because size_of is already in the Rust 2024 prelude. Removing the qualification in this one-line test change makes the check pass on the base branch.