feat: update requesters and improve provider selection UI

- Added `litellm_provider` field to various requesters' YAML configurations.
- Removed obsolete Python requester files for OpenRouter, PPIO, QHAIGC, ShengSuanYun, SiliconFlow, Space, TokenPony, VolcArk, and Xai.
- Introduced new requesters for Tencent and Together AI with corresponding YAML configurations and SVG icons.
- Enhanced the ProviderForm component to include a searchable dropdown for selecting providers, improving user experience.
- Updated localization files to include search provider text for both English and Chinese.

src/langbot/pkg/provider/modelmgr/requesters/mistralchatcmpl.yaml

@@ -0,0 +1,30 @@
+apiVersion: v1
+kind: LLMAPIRequester
+metadata:
+  name: mistral-chat-completions
+  label:
+    en_US: Mistral AI
+    zh_Hans: Mistral AI
+  icon: mistral.svg
+spec:
+  litellm_provider: mistral
+  config:
+  - name: base_url
+    label:
+      en_US: Base URL
+      zh_Hans: 基础 URL
+    type: string
+    required: true
+    default: https://api.mistral.ai/v1
+  - name: timeout
+    label:
+      en_US: Timeout
+      zh_Hans: 超时时间
+    type: integer
+    required: true
+    default: 120
+  support_type:
+  - llm
+  - text-embedding
+  - rerank
+  provider_category: manufacturer