fix: remove ANTHROPIC_REASONING_MODEL to decouple thinking from model selection (#2081)

ANTHROPIC_REASONING_MODEL was a non-official env var that forced all
requests with thinking params to use a single "reasoning model",
overriding the user's /model selection. Since new Claude Code versions
send adaptive thinking by default, this caused /model to silently fail.

- Remove reasoning_model field and has_thinking_enabled() from model_mapper
- Simplify map_model() to pure type-based matching (haiku/sonnet/opus)
- Remove reasoning model UI field from provider form
- Retain ANTHROPIC_REASONING_MODEL in ENV_EXCLUDES and override-key
  cleanup lists so legacy configs don't leak into common config

src/i18n/locales/en.json

@@ -820,7 +820,6 @@
     "visitWebsite": "Visit {{url}}",
     "anthropicModel": "Main Model",
     "anthropicSmallFastModel": "Fast Model",
-    "anthropicReasoningModel": "Reasoning Model (Thinking)",
     "apiFormat": "API Format",
     "apiFormatHint": "Select the input format for the provider's API",
     "fullUrlLabel": "Full URL",

src/i18n/locales/ja.json

@@ -820,7 +820,6 @@
     "visitWebsite": "{{url}} を開く",
     "anthropicModel": "メインモデル",
     "anthropicSmallFastModel": "高速モデル",
-    "anthropicReasoningModel": "推論モデル（Thinking）",
     "apiFormat": "API フォーマット",
     "apiFormatHint": "プロバイダー API の入力フォーマットを選択",
     "fullUrlLabel": "フル URL",

src/i18n/locales/zh.json

@@ -821,7 +821,6 @@
     "visitWebsite": "访问 {{url}}",
     "anthropicModel": "主模型",
     "anthropicSmallFastModel": "快速模型",
-    "anthropicReasoningModel": "推理模型 (Thinking)",
     "apiFormat": "API 格式",
     "apiFormatHint": "选择供应商 API 的输入格式",
     "fullUrlLabel": "完整 URL",