[codex] fix request navigation

[shanejonas]

What changed

• Explain when an active filter hides every request.
• Clear stale filters when sessions change.
• Open the selected response with Enter from Requests.
• Preserve Markdown copying on Ctrl-B y.

Why

A request could complete successfully while the list looked empty, and Enter copied the table instead of opening the response the user selected.

Verification

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --locked
• cargo package --locked