docs(api): document add_messages batch endpoint

[r266-tech]

PR #2213 added POST /api/v1/sessions/{session_id}/messages/batch and the batch_add_messages() SDK method, but docs/{en,zh}/api/05-sessions.md only document add_message() (single). This adds a parallel add_messages() section (EN + ZH) covering the HTTP endpoint, Python SDK usage, parameters (100-message cap, same per-message fields as add_message()), and the added count in the response.

Source: #2213 (sponge225). Verified params/route/SDK signature against openviking/server/routers/sessions.py (BatchAddMessageRequest, batch_add_messages) and openviking_cli/client/{base,http,sync_http}.py.

[github-actions]

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🎫 Ticket compliance analysis ✅ 2213 - Fully compliant Compliant requirements: Added add_messages() documentation to both EN/ZH docs Included HTTP endpoint, SDK examples, parameters, response

⏱️ Estimated effort to review: 1 🔵⚪⚪⚪⚪

🏅 Score: 100

🧪 No relevant tests

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[r266-tech]

Superseded by #2218, which was merged earlier today and added a full batch_add_messages() section to the same docs/{en,zh}/api/05-sessions.md files — documenting the identical POST /api/v1/sessions/{session_id}/messages/batch endpoint. Closing this as redundant since the endpoint is now documented on main. Thanks!