sync(aichat): add aichat service directory from Docs b3b2922

[Copilot]

Upstream AceDataCloud/Docs updated the aichat service (commit b3b2922). The aichat/ subdirectory did not exist in this monorepo.

New files

• aichat/README.md — service overview, full supported model list (GPT-5, GPT-4o, o-series, DeepSeek, Grok, GLM), quick-start snippet, and endpoint table
• aichat/docs/aichat_conversations_api_integration_guide.md — English integration guide translated from the upstream MDX source, covering:
  • Single-turn Q&A
  • Stateful multi-turn conversation (stateful: true + id round-trip)
  • Streaming (accept: application/x-ndjson) with Python / Node.js / Java examples
  • Model presets (preset / system prompt)
  • Image recognition via references
  • Web-browsing models
  • Full parameter + response field reference tables

Updated files

• README.md — added aichat/ row to the monorepo API table
• sync.yaml — registered aichat → AceDataCloud/AIChatAPI sync mapping

Quick-start (from guide)

curl -X POST 'https://api.acedata.cloud/aichat/conversations' \
  -H 'authorization: ******' \
  -H 'content-type: application/json' \
  -d '{"model": "gpt-4o", "question": "What is the capital of France?"}'
# {"answer": "The capital of France is Paris."}

[acedatacloud-dev]

Auto-closed during PR backlog cleanup: superseded sync (+445/-0) — content covered by next dispatch against current Docs. Root cause was a broken auto-merge in sync-from-docs.yml (now fixed in PR #143). The fix lands a pre-cleanup step + reliable completion signal so future dispatches will not leak PRs. If this PR contained content not yet in main, the next docs-updated dispatch will re-create it via the working pipeline.