fix(deepseek): keep Anthropic-only tool types when forwarding to /anthropic/v1/messages

DeepSeek's Anthropic-compatible endpoint accepts only the built-in
web_search_20250305 / web_search_20260209 tools and rejects client-defined
`custom` tools (MCP / Read / Bash) with HTTP 400 "unknown variant `custom`".
The generic non-Claude filter in prepareClaudeRequest dropped the offending
tools but also dropped the web_search_* ones DeepSeek does accept.

- Add an opt-in per-provider transport quirk `claudeSupportedToolTypes`; when
  declared it becomes a strict allow-list for Anthropic tool `type` values
- Stop stripping the `type` discriminator from surviving tools under that
  quirk, since DeepSeek needs it to route built-ins
- Declare the quirk on the deepseek transport with the two web_search_* types
- Providers without the quirk keep the previous filter and normalisation
  behaviour byte-for-byte; openai-format targets never reach this path

open-sse/providers/registry/deepseek.js

@@ -25,6 +25,21 @@ export default {
     reasoningInject: {
       scope: "all",
     },
+    quirks: {
+      // DeepSeek's Anthropic-compatible endpoint
+      // (https://api.deepseek.com/anthropic/v1/messages) accepts ONLY the
+      // built-in web_search_* tools and rejects client-defined `custom` tools
+      // (MCP / Read / Bash / etc.) with HTTP 400
+      //   "tools[0]: unknown variant `custom`, expected
+      //    `web_search_20250305` or `web_search_20260209`".
+      //
+      // Declaring this whitelist makes prepareClaudeRequest() forward only
+      // web_search_* tools and strip everything else before sending, so MCP /
+      // function tools are dropped instead of failing the whole request.
+      // DeepSeek's OpenAI-compatible transport is unaffected (targetFormat
+      // there is "openai", not "claude", so prepareClaudeRequest is not run).
+      claudeSupportedToolTypes: ["web_search_20250305", "web_search_20260209"],
+    },
   },
   // Multi-endpoint: pick the transport matching client sourceFormat to skip translation.
   transports: [