test: fold settings.rs term_program + no_animations guards into lock_test_env

PR #1365 (cherry-picked into v0.8.28) introduced
`term_program_test_guard` as a fresh module-local
`static Mutex<()>`, mirroring the existing
`no_animations_test_guard`. Both serialize their own family of
tests but not with each other — so under cargo's parallel runner,
a `NO_ANIMATIONS=1` leak from one family lands in the env at
the exact moment a `TERM_PROGRAM=iTerm.app` test calls the shared
`apply_env_overrides`, flipping `low_motion` to true and failing
`non_vscode_term_program_does_not_force_low_motion`.

Both guards now return `crate::test_support::lock_test_env()`
(the same fold the v0.8.28 test-stabilization commit applied to
the EnvGuard family in `commands/config.rs`, `commands/network.rs`,
and `tools/recall_archive.rs`). This serializes the two test
groups with each other and with every other env-mutating test in
the suite, eliminating the cross-test env-var race.

`save_api_key_for_openrouter_writes_provider_table` was failing
intermittently for the same reason — a concurrent env mutation
in an unrelated test was clobbering HOME / DEEPSEEK_CONFIG_PATH
in the window between our `EnvGuard::new` and
`save_api_key_for`'s `default_config_path()` read. With the
broader serialization in place, the race window closes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/tui/src/settings.rs

@@ -826,10 +826,13 @@ mod tests {
 
     /// Tests that mutate process-global `NO_ANIMATIONS` serialise
     /// through this guard so the cargo parallel runner doesn't
-    /// observe interleaved overrides.
+    /// observe interleaved overrides. Uses the process-wide test env
+    /// lock so this serializes with the TERM_PROGRAM tests too —
+    /// otherwise a `NO_ANIMATIONS=1` leak from this test family can
+    /// flip a concurrent `TERM_PROGRAM=iTerm` test's `low_motion`
+    /// assertion through the shared `apply_env_overrides` path.
     fn no_animations_test_guard() -> std::sync::MutexGuard<'static, ()> {
-        static GUARD: std::sync::Mutex<()> = std::sync::Mutex::new(());
-        GUARD.lock().unwrap_or_else(|e| e.into_inner())
+        crate::test_support::lock_test_env()
     }
 
     #[test]
@@ -906,9 +909,13 @@ mod tests {
     }
 
     /// Serialise tests that mutate `TERM_PROGRAM` through this guard.
+    /// Uses the process-wide test env lock so this serializes not just
+    /// with itself but with every other env-mutating test in the suite
+    /// — otherwise a concurrent test that calls `Settings::default()`
+    /// can read whatever value our two `set_var`s have raced into the
+    /// env at that instant.
     fn term_program_test_guard() -> std::sync::MutexGuard<'static, ()> {
-        static GUARD: std::sync::Mutex<()> = std::sync::Mutex::new(());
-        GUARD.lock().unwrap_or_else(|e| e.into_inner())
+        crate::test_support::lock_test_env()
     }
 
     #[test]