Render multiline hook output in TUI

[abhinav-oai]

Why

Fixes #24529. Completed hook output in the TUI rendered each HookOutputEntry as one ratatui line, so explicit newlines inside hook output were not shown as separate transcript rows. That made multiline SessionStart.additionalContext hard to inspect even though the model-facing context path preserved the original text.

What

• Split completed hook output entries on explicit newlines before rendering them in codex-rs/tui/src/history_cell/hook_cell.rs.
• Keep the hook output prefix, such as hook context: or warning:, on the first physical line only.
• Preserve explicit blank lines and render continuation lines with the hook body indent.
• Add unit coverage for multiline context and warning output, plus a chatwidget snapshot regression for SessionStart history output.

Testing

• cargo nextest run -p codex-tui completed_hook_multiline hook_completed_before_reveal_renders_completed_without_running_flash
• just argument-comment-lint -p codex-tui -- --ignore-rust-version --lib --tests

[fcoury-oai]

Smoke tested and works as expected, good looks good! 👍

Approved.