tui: remove the voice-input crate feature (#16467)

## Why

`voice-input` is the only remaining TUI crate feature, but it is also a
default feature and nothing in the workspace selects it explicitly. In
practice it is just acting as a proxy for platform support, which is
better expressed with target-specific dependencies and cfgs.

## What changed

- remove the `voice-input` feature from `codex-tui`
- make `cpal` a normal non-Linux target dependency
- replace the feature-based voice and audio cfgs with pure
Linux-vs-non-Linux cfgs
- shrink the workspace-manifest verifier allowlist to remove the
remaining `codex-tui` exception

## How tested

- `python3 .github/scripts/verify_cargo_workspace_manifests.py`
- `cargo test -p codex-tui`
- `just bazel-lock-check`
- `just argument-comment-lint -p codex-tui`

.github/scripts/verify_cargo_workspace_manifests.py

@@ -30,18 +30,8 @@ MANIFEST_FEATURE_EXCEPTIONS = {
     "codex-rs/otel/Cargo.toml": {
         "disable-default-metrics-exporter": (),
     },
-    "codex-rs/tui/Cargo.toml": {
-        "default": ("voice-input",),
-        "voice-input": ("dep:cpal",),
-    },
-}
-OPTIONAL_DEPENDENCY_EXCEPTIONS = {
-    (
-        "codex-rs/tui/Cargo.toml",
-        'target.cfg(not(target_os = "linux")).dependencies',
-        "cpal",
-    ),
 }
+OPTIONAL_DEPENDENCY_EXCEPTIONS = set()
 INTERNAL_DEPENDENCY_FEATURE_EXCEPTIONS = {
     (
         "codex-rs/core/Cargo.toml",