feat(codex): add default model field to provider form

Expose the top-level `model` key of config.toml as an editable field in
the Codex provider form, so users can switch to newly released models
(e.g. gpt-5.6) without waiting for a preset update — preset updates only
affect newly added providers, existing ones keep their saved TOML.

The field syncs bidirectionally with the TOML editor (mirroring the
base-URL pattern), suggests models from the mapping catalog plus the
provider's /models endpoint, and offers a one-click "add to mapping"
action when the value is outside a non-empty catalog. Hidden for
official providers.

Details:

- Save-time catalog sync now only backfills the first mapping row into
  the top-level model when the field was left empty, so the explicit
  field wins over the implicit row-0 sync
- Escape model names (and base_url) with TOML basic-string escaping and
  strip control characters from field input: /models ids are remote
  data and unescaped interpolation could inject config.toml lines
  (e.g. a forged [mcp_servers.*] command)
- The strict model-line matcher now recognizes escaped output, empty
  strings and single-quoted literals, keeping extract/set round-trips
  stable; deliberately no key-only loose matching (it would misfire on
  assignment-looking text inside multiline strings)
- Invalidate the fetched model list whenever the request identity
  changes (base URL, full-URL toggle, API key, custom UA) and discard
  in-flight responses via a sequence guard, so the dropdown never shows
  a previous provider's models
- i18n for zh/en/ja/zh-TW

src/i18n/locales/en.json

@@ -1307,6 +1307,11 @@
     "autoCompactLimitHint": "Auto-compacts history when context reaches this token limit",
     "modelMetadataAdvanced": "Model Metadata Advanced Options",
     "modelMetadataAdvancedHint": "Overrides Codex metadata for unknown models; clearing a field removes the matching config.toml entry.",
+    "defaultModelLabel": "Default Model",
+    "defaultModelPlaceholder": "e.g. gpt-5.6",
+    "defaultModelHint": "The model Codex requests by default; change it anytime without waiting for a preset update. If left empty while model mapping is configured, the first mapping row is used.",
+    "defaultModelNotInCatalog": "This model is not in the model mapping, so the Codex /model menu won't list it (direct requests still work).",
+    "addToModelMapping": "Add to mapping",
     "modelMappingTitle": "Model Mapping",
     "modelMappingHint": "Generates Codex model_catalog_json so /model can show these third-party model names; entries are saved exactly as listed. Codex must be restarted to refresh the model list after changes.",
     "addCatalogModel": "Add Model",

src/i18n/locales/ja.json

@@ -1307,6 +1307,11 @@
     "autoCompactLimitHint": "コンテキストトークン数がこのしきい値に達すると履歴を自動圧縮",
     "modelMetadataAdvanced": "モデルメタデータ詳細オプション",
     "modelMetadataAdvancedHint": "未知のモデル向けに Codex メタデータを上書きします。空にすると対応する config.toml 設定を削除します。",
+    "defaultModelLabel": "デフォルトモデル",
+    "defaultModelPlaceholder": "例: gpt-5.6",
+    "defaultModelHint": "Codex がデフォルトでリクエストするモデルです。プリセットの更新を待たずにいつでも変更できます。空欄のままモデルマッピングを設定している場合は、マッピングの先頭行が使われます。",
+    "defaultModelNotInCatalog": "このモデルはモデルマッピングにないため、Codex の /model メニューには表示されません（直接リクエストは有効です）。",
+    "addToModelMapping": "マッピングに追加",
     "modelMappingTitle": "モデルマッピング",
     "modelMappingHint": "Codex model_catalog_json を生成し、/model コマンドでこれらの第三者モデル名を表示できるようにします。リストの内容はそのまま保存されます。変更後、モデルリストを更新するには Codex の再起動が必要です。",
     "addCatalogModel": "モデルを追加",

src/i18n/locales/zh-TW.json

@@ -1273,6 +1273,11 @@
     "maxOutputTokensLabel": "最大輸出 tokens",
     "maxOutputTokensPlaceholder": "留空則使用預設 8192",
     "maxOutputTokensHint": "Codex 不會把 model_max_output_tokens 寫進請求體，預設上限 8192 容易在長回答或深度思考時被截斷（stop_reason=max_tokens）。此處設定會作為 Anthropic 的 max_tokens 覆蓋請求值。請勿超過該模型／閘道的真實輸出上限，否則可能 400。留空使用預設 8192。",
+    "defaultModelLabel": "預設模型",
+    "defaultModelPlaceholder": "例如: gpt-5.6",
+    "defaultModelHint": "Codex 預設請求的模型，隨時可改，無需等待軟體更新供應商範本。留空且設定了模型映射時，預設使用映射第一行。",
+    "defaultModelNotInCatalog": "該模型不在模型映射中，Codex 的 /model 選單不會列出它（直接請求仍然有效）。",
+    "addToModelMapping": "加入映射",
     "modelMappingTitle": "模型映射",
     "modelMappingHint": "產生 Codex model_catalog_json，讓 /model 指令顯示這些第三方模型名；表中條目按填寫內容原樣儲存。修改後需要重新啟動 Codex 才能重新整理模型清單。",
     "addCatalogModel": "新增模型",

src/i18n/locales/zh.json

@@ -1307,6 +1307,11 @@
     "autoCompactLimitHint": "上下文 token 数达到此阈值时自动压缩历史",
     "modelMetadataAdvanced": "模型元数据高级选项",
     "modelMetadataAdvancedHint": "用于未知模型的 Codex 元数据覆盖；清空字段会从 config.toml 删除对应配置。",
+    "defaultModelLabel": "默认模型",
+    "defaultModelPlaceholder": "例如: gpt-5.6",
+    "defaultModelHint": "Codex 默认请求的模型，随时可改，无需等待预设更新。留空且配置了模型映射时，默认使用映射第一行。",
+    "defaultModelNotInCatalog": "该模型不在模型映射中，Codex 的 /model 菜单不会列出它（直接请求仍然有效）。",
+    "addToModelMapping": "加入映射",
     "modelMappingTitle": "模型映射",
     "modelMappingHint": "生成 Codex model_catalog_json，让 /model 命令显示这些第三方模型名；表中条目按填写内容原样保存。修改后需要重启 Codex 才能刷新模型列表。",
     "addCatalogModel": "添加模型",