Fix Claude CLI reasoning effort fallback

[suninrain086]

Problem

Using Claude CLI against the gateway initially failed with the following upstream Copilot error:

say hello in one sentence
  ⎿  API Error: 400 {"type":"error","error":{"type":"api_error","message":"Copilot API error: 400
     {\"error\":{\"message\":\"output_config.effort \\\"xhigh\\\" is not supported by model claude-opus-4.7; supported values:
     [medium]\",\"code\":\"invalid_reasoning_effort\"}}\n"}}

The root cause is that native /v1/messages requests can forward Claude CLI reasoning effort settings that Copilot rejects for a given model, and the gateway previously surfaced that 400 directly back to the client.

Summary

• accept xhigh in Anthropic output_config.effort typing
• detect native /v1/messages invalid_reasoning_effort responses from Copilot
• retry the request with the nearest supported effort instead of surfacing a 400 back to Claude CLI
• add regression coverage for the xhigh to medium fallback path

Testing

• deno test
• Result: 187 passed / 0 failed
• Passed test suites:
  • src/app-control_test.ts
  • src/lib/models-cache_test.ts
  • src/lib/probe_test.ts
  • src/lib/sse-reassemble_test.ts
  • src/lib/translate/anthropic-to-responses-stream_test.ts
  • src/lib/translate/chat-to-messages_test.ts
  • src/lib/translate/messages-to-chat-stream_test.ts
  • src/lib/translate/messages-to-chat_test.ts
  • src/lib/translate/responses-request_test.ts
  • src/middleware/usage_test.ts
  • src/routes/chat-completions_test.ts
  • src/routes/count-tokens_test.ts
  • src/routes/data-transfer_test.ts
  • src/routes/messages_test.ts
  • src/routes/responses_test.ts
  • src/ui/dashboard_test.ts