TUI: split history cells into focused modules

[etraut-openai]

Why

codex-rs/tui/src/history_cell.rs had become the dumping ground for transcript rendering: the shared trait, common helpers, and the concrete cells for messages, plans, MCP/search, notices, patches, approvals, session chrome, and separators all lived together. That made small transcript changes require reopening a very large file and made ownership less obvious.

What changed

• Replaced the monolithic history_cell.rs with a history_cell/ module tree organized by concern.
• Kept the existing crate::history_cell::* surface stable through re-exports in history_cell/mod.rs.
• Moved the existing render coverage into history_cell/tests.rs.

Reviewer notes

• This PR is intentionally mechanical in mature — existing code and tests moving into files that match their concern.
• The snapshot files under codex-rs/tui/src/history_cell/snapshots/ moved with the extracted test module. insta resolves these unnamed snapshots relative to the source file that declares them, so this is path churn only; snapshot contents were not updated.
• The small non-mechanical seam edits are limited to split fallout: sibling-module visibility for shared cell containers, moving approval-specific exec-snippet helpers beside approvals, fixing the separator module path, and keeping a couple of existing test helpers reachable after extraction.