feat: unify Codex third-party providers into stable "custom" history bucket

Codex filters resume history by `model_provider`, so switching between
provider-specific ids like `rightcode` and `aihubmix` made past sessions
appear to vanish. Collapse all third-party providers into a single
stable bucket so cross-switch history stays visible.

- Normalize live `model_provider` to "custom" on every Codex write
  (reserved built-in ids like openai/ollama are preserved).
- Add device-level one-shot migration that rewrites historical JSONL
  session files and the `state_5.sqlite` threads table from legacy
  provider ids into the "custom" bucket. Backs up originals under
  `~/.cc-switch/backups/codex-history-provider-migration-v1/` and uses
  the SQLite Backup API for the state DB.
- Record completion in `settings.json` under `localMigrations` so the
  migration is strictly idempotent across launches.
- Update Codex provider preset templates to emit `model_provider = "custom"`
  out of the box.

src-tauri/tests/provider_service.rs

@@ -309,8 +309,8 @@ requires_openai_auth = true
 
     assert_eq!(
         parsed.get("model_provider").and_then(|v| v.as_str()),
-        Some("rightcode"),
-        "live Codex model_provider should stay stable so resume history remains visible"
+        Some("custom"),
+        "live Codex third-party model_provider should use the CC Switch history bucket"
     );
 
     let model_providers = parsed
@@ -323,7 +323,7 @@ requires_openai_auth = true
     );
     assert_eq!(
         model_providers
-            .get("rightcode")
+            .get("custom")
             .and_then(|v| v.get("base_url"))
             .and_then(|v| v.as_str()),
         Some("https://aihubmix.example/v1"),