chore(core) personality migration tests (#10650)

## Summary
Adds additional tests for personality edge cases

## Testing
- [x] These are tests

codex-rs/core/tests/suite/personality_migration.rs

@@ -42,6 +42,16 @@ async fn write_archived_session_with_user_event(codex_home: &Path) -> io::Result
     write_rollout_with_user_event(&dir, thread_id).await
 }
 
+async fn write_session_with_meta_only(codex_home: &Path) -> io::Result<()> {
+    let thread_id = ThreadId::new();
+    let dir = codex_home
+        .join(SESSIONS_SUBDIR)
+        .join("2025")
+        .join("01")
+        .join("01");
+    write_rollout_with_meta_only(&dir, thread_id).await
+}
+
 async fn write_rollout_with_user_event(dir: &Path, thread_id: ThreadId) -> io::Result<()> {
     tokio::fs::create_dir_all(&dir).await?;
     let file_path = dir.join(format!("rollout-{TEST_TIMESTAMP}-{thread_id}.jsonl"));
@@ -83,6 +93,40 @@ async fn write_rollout_with_user_event(dir: &Path, thread_id: ThreadId) -> io::R
     Ok(())
 }
 
+async fn write_rollout_with_meta_only(dir: &Path, thread_id: ThreadId) -> io::Result<()> {
+    tokio::fs::create_dir_all(&dir).await?;
+    let file_path = dir.join(format!("rollout-{TEST_TIMESTAMP}-{thread_id}.jsonl"));
+    let mut file = tokio::fs::File::create(&file_path).await?;
+
+    let session_meta = SessionMetaLine {
+        meta: SessionMeta {
+            id: thread_id,
+            forked_from_id: None,
+            timestamp: TEST_TIMESTAMP.to_string(),
+            cwd: std::path::PathBuf::from("."),
+            originator: "test_originator".to_string(),
+            cli_version: "test_version".to_string(),
+            source: SessionSource::Cli,
+            model_provider: None,
+            base_instructions: None,
+            dynamic_tools: None,
+        },
+        git: None,
+    };
+    let meta_line = RolloutLine {
+        timestamp: TEST_TIMESTAMP.to_string(),
+        item: RolloutItem::SessionMeta(session_meta),
+    };
+
+    let meta_json = serde_json::to_string(&meta_line)?;
+    file.write_all(format!("{meta_json}\n").as_bytes()).await?;
+    Ok(())
+}
+
+fn parse_config_toml(contents: &str) -> io::Result<ConfigToml> {
+    toml::from_str(contents).map_err(|err| io::Error::new(io::ErrorKind::InvalidData, err))
+}
+
 #[tokio::test]
 async fn migration_marker_exists_no_sessions_no_change() -> io::Result<()> {
     let temp = TempDir::new()?;
@@ -135,6 +179,138 @@ async fn no_marker_sessions_sets_personality() -> io::Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn no_marker_sessions_preserves_existing_config_fields() -> io::Result<()> {
+    let temp = TempDir::new()?;
+    write_session_with_user_event(temp.path()).await?;
+    tokio::fs::write(temp.path().join("config.toml"), "model = \"gpt-5-codex\"\n").await?;
+    let config_toml = read_config_toml(temp.path()).await?;
+
+    let status = maybe_migrate_personality(temp.path(), &config_toml).await?;
+
+    assert_eq!(status, PersonalityMigrationStatus::Applied);
+    let persisted = read_config_toml(temp.path()).await?;
+    assert_eq!(persisted.model, Some("gpt-5-codex".to_string()));
+    assert_eq!(persisted.personality, Some(Personality::Pragmatic));
+    Ok(())
+}
+
+#[tokio::test]
+async fn no_marker_meta_only_rollout_is_treated_as_no_sessions() -> io::Result<()> {
+    let temp = TempDir::new()?;
+    write_session_with_meta_only(temp.path()).await?;
+
+    let status = maybe_migrate_personality(temp.path(), &ConfigToml::default()).await?;
+
+    assert_eq!(status, PersonalityMigrationStatus::SkippedNoSessions);
+    assert_eq!(
+        tokio::fs::try_exists(temp.path().join(PERSONALITY_MIGRATION_FILENAME)).await?,
+        true
+    );
+    assert_eq!(
+        tokio::fs::try_exists(temp.path().join("config.toml")).await?,
+        false
+    );
+    Ok(())
+}
+
+#[tokio::test]
+async fn no_marker_explicit_global_personality_skips_migration() -> io::Result<()> {
+    let temp = TempDir::new()?;
+    write_session_with_user_event(temp.path()).await?;
+    let config_toml = parse_config_toml("personality = \"friendly\"\n")?;
+
+    let status = maybe_migrate_personality(temp.path(), &config_toml).await?;
+
+    assert_eq!(
+        status,
+        PersonalityMigrationStatus::SkippedExplicitPersonality
+    );
+    assert_eq!(
+        tokio::fs::try_exists(temp.path().join(PERSONALITY_MIGRATION_FILENAME)).await?,
+        true
+    );
+    assert_eq!(
+        tokio::fs::try_exists(temp.path().join("config.toml")).await?,
+        false
+    );
+    Ok(())
+}
+
+#[tokio::test]
+async fn no_marker_profile_personality_skips_migration() -> io::Result<()> {
+    let temp = TempDir::new()?;
+    write_session_with_user_event(temp.path()).await?;
+    let config_toml = parse_config_toml(
+        r#"
+profile = "work"
+
+[profiles.work]
+personality = "friendly"
+"#,
+    )?;
+
+    let status = maybe_migrate_personality(temp.path(), &config_toml).await?;
+
+    assert_eq!(
+        status,
+        PersonalityMigrationStatus::SkippedExplicitPersonality
+    );
+    assert_eq!(
+        tokio::fs::try_exists(temp.path().join(PERSONALITY_MIGRATION_FILENAME)).await?,
+        true
+    );
+    assert_eq!(
+        tokio::fs::try_exists(temp.path().join("config.toml")).await?,
+        false
+    );
+    Ok(())
+}
+
+#[tokio::test]
+async fn marker_short_circuits_invalid_profile_resolution() -> io::Result<()> {
+    let temp = TempDir::new()?;
+    tokio::fs::write(temp.path().join(PERSONALITY_MIGRATION_FILENAME), "v1\n").await?;
+    let config_toml = parse_config_toml("profile = \"missing\"\n")?;
+
+    let status = maybe_migrate_personality(temp.path(), &config_toml).await?;
+
+    assert_eq!(status, PersonalityMigrationStatus::SkippedMarker);
+    Ok(())
+}
+
+#[tokio::test]
+async fn invalid_selected_profile_returns_error_and_does_not_write_marker() -> io::Result<()> {
+    let temp = TempDir::new()?;
+    let config_toml = parse_config_toml("profile = \"missing\"\n")?;
+
+    let err = maybe_migrate_personality(temp.path(), &config_toml)
+        .await
+        .expect_err("missing profile should fail");
+
+    assert_eq!(err.kind(), io::ErrorKind::InvalidData);
+    assert_eq!(
+        tokio::fs::try_exists(temp.path().join(PERSONALITY_MIGRATION_FILENAME)).await?,
+        false
+    );
+    Ok(())
+}
+
+#[tokio::test]
+async fn applied_migration_is_idempotent_on_second_run() -> io::Result<()> {
+    let temp = TempDir::new()?;
+    write_session_with_user_event(temp.path()).await?;
+
+    let first_status = maybe_migrate_personality(temp.path(), &ConfigToml::default()).await?;
+    let second_status = maybe_migrate_personality(temp.path(), &ConfigToml::default()).await?;
+
+    assert_eq!(first_status, PersonalityMigrationStatus::Applied);
+    assert_eq!(second_status, PersonalityMigrationStatus::SkippedMarker);
+    let persisted = read_config_toml(temp.path()).await?;
+    assert_eq!(persisted.personality, Some(Personality::Pragmatic));
+    Ok(())
+}
+
 #[tokio::test]
 async fn no_marker_archived_sessions_sets_personality() -> io::Result<()> {
     let temp = TempDir::new()?;