fix(ai-proxy): update anthropic messages endpoint path for qwen provider

[Jing-ze]

Ⅰ. Describe what this PR did

Update the Qwen provider's Anthropic-compatible API endpoint from the legacy path /api/v2/apps/claude-code-proxy/v1/messages to the new official path /apps/anthropic/v1/messages. The new endpoint (/apps/anthropic) as documented in the Bailian Anthropic API compatibility documentation.

Ⅱ. Does this pull request fix one issue?

N/A

Ⅲ. Why don't you add test cases (unit test/integration test)?

This is a one-line constant change (URL path string).

Ⅳ. Describe how to verify it

curl -X POST "https://dashscope.aliyuncs.com/apps/anthropic/v1/messages" \
  -H "Content-Type: application/json" \
  -H "x-api-key: ${DASHSCOPE_API_KEY}" \
  -d '{"model":"qwen-plus","max_tokens":100,"messages":[{"role":"user","content":"Say hello"}]}'

Ⅴ. Special notes for reviews

• Only the qwenAnthropicMessagesPath constant is changed. No logic changes.

Ⅵ. AI Coding Tool Usage Checklist (if applicable)

Please check all applicable items:

• For regular updates/changes (not new plugins):
  • I have provided the prompts/instructions I gave to the AI Coding tool below
  • I have included the AI Coding summary below

AI Coding Prompts (for regular updates)

AI Coding Summary

• Key decision: Only the URL path constant needed updating; the auth, host, and protocol handling logic are all path-agnostic.
• Major change: qwenAnthropicMessagesPath changed from /api/v2/apps/claude-code-proxy/v1/messages to /apps/anthropic/v1/messages in provider/qwen.go.
• Verification: Live-tested through a Higress gateway with the compiled WASM plugin against real Dashscope API, confirming streaming, thinking mode, system prompts, and multiple auth methods all work correctly with the new endpoint.

[rinfx]

LGTM