refactor(codex): unify custom model_provider routing key to "custom"

- Always emit `model_provider = "custom"` from deep link, UniversalProvider, and the universal form modal so future writes share one stable routing key.
- Add `codex_provider_template_v1` local migration that rewrites legacy keys (aihubmix/ccswitch/...) under `[model_providers.custom]`, updates profile refs, and backs up the original settings_config under `~/.cc-switch/backups/<timestamp>/providers/`.
- Tighten history migration source detection to a whitelist plus `[model_providers.<id>]` existence check so user-authored keys are never rewritten in jsonl/state DB.
- Encode deep link name/model/endpoint through `toml_edit::Value` so display names containing quotes or backslashes no longer break the generated config.toml.
- Stabilize provider settings backup filename hash with Sha256 (was process-random SipHash).

src-tauri/src/codex_config.rs

@@ -172,17 +172,6 @@ pub(crate) fn is_custom_codex_model_provider_id(id: &str) -> bool {
             .any(|reserved| reserved.eq_ignore_ascii_case(id))
 }
 
-pub(crate) fn stable_codex_model_provider_id_from_config(config_text: &str) -> Option<String> {
-    let doc = config_text.parse::<DocumentMut>().ok()?;
-    let provider_id = active_codex_model_provider_id(&doc)?;
-
-    if is_custom_codex_model_provider_id(&provider_id) {
-        Some(provider_id)
-    } else {
-        None
-    }
-}
-
 /// Write only Codex `config.toml` for provider switching.
 ///
 /// Codex login state lives in `auth.json`; provider routing, endpoint, model,