ci: stop running rust CI with --all-features (#16473)

## Why

Now that workspace crate features have been removed and
`.github/scripts/verify_cargo_workspace_manifests.py` hard-bans new
ones, Rust CI should stop building and testing with `--all-features`.

Keeping `--all-features` in CI no longer buys us meaningful coverage for
`codex-rs`, but it still makes the workflow look like we rely on Cargo
feature permutations that we are explicitly trying to eliminate. It also
leaves stale examples in the repo that suggest `--all-features` is a
normal or recommended way to run the workspace.

## What changed

- removed `--all-features` from the Rust CI `cargo chef cook`, `cargo
clippy`, and `cargo nextest` invocations in
`.github/workflows/rust-ci-full.yml`
- updated the `just test` guidance in `justfile` to reflect that
workspace crate features are banned and there should be no need to add
`--all-features`
- updated the multiline command example and snapshot in
`codex-rs/tui/src/history_cell.rs` to stop rendering `cargo test
--all-features --quiet`
- tightened the verifier docstring in
`.github/scripts/verify_cargo_workspace_manifests.py` so it no longer
talks about temporary remaining exceptions

## How tested

- `python3 .github/scripts/verify_cargo_workspace_manifests.py`
- `cargo test -p codex-tui`

.github/scripts/verify_cargo_workspace_manifests.py

@@ -6,8 +6,7 @@ Checks:
 - Crates inherit `[workspace.package]` metadata.
 - Crates opt into `[lints] workspace = true`.
 - Crate names follow the codex-rs directory naming conventions.
-- Workspace manifests do not introduce new workspace crate feature toggles
-  while the remaining exceptions are being removed.
+- Workspace manifests do not introduce workspace crate feature toggles.
 """
 
 from __future__ import annotations