[BUG] MCP notifications/claude/channel not delivered under /anthropic endpoint (Claude Code)

## Describe the bug

When using Claude Code with `ANTHROPIC_BASE_URL=https://api.deepseek.com/anthropic`, MCP server notifications (`notifications/claude/channel`) are **not processed** by the Claude Code harness. The notifications are correctly written to stdout by the MCP server (verified via strace/debug logging), but Claude Code does not display them as `` tags in the conversation.

This breaks **Telegram/WeChat channel messaging** and **permission relay** features in Claude Code.

## Steps to reproduce

1. Set up Claude Code with DeepSeek backend:
```
ANTHROPIC_BASE_URL=https://api.deepseek.com/anthropic
ANTHROPIC_MODEL=deepseek-v4-pro[1m]
```
2. Install Telegram channel plugin:
```
claude --channels plugin:telegram@claude-plugins-official
```
3. Send a DM to the configured Telegram bot
4. Observe: MCP server receives the message and sends `notifications/claude/channel` via stdout (confirmed with debug logging)
5. **Actual**: No `` tag appears in Claude Code conversation
6. **Expected**: Message should appear as ``

## Environment

- Claude Code version: 2.1.181
- MCP SDK version: 1.29.0
- DeepSeek API endpoint: `/anthropic` (Anthropic compatibility)
- OS: Linux 5.15.0
- Channel plugins tested: Telegram (claude-plugins-official), WeChat (m1heng-plugins)

## Impact

- Channel messages (Telegram/WeChat) cannot be delivered in real-time
- Permission relay via inline buttons does not work
- Users must use polling-based workarounds (read_inbox tool)
- Affects all Claude Code channel plugins under DeepSeek backend

## Workaround

A `read_inbox` MCP tool + cron-based polling provides partial functionality, but permission approval still requires terminal access.

## Related

- May be related to #1369 / #1370 (Anthropic endpoint compatibility issues)
- The same MCP notification mechanism works correctly under the official Anthropic API (unverified but likely)

## Note

This issue may need to be addressed in the `/anthropic` compatibility layer. The MCP `notifications/claude/channel` notification is a standard part of the Claude Code channel protocol.

Co-Authored-By: Claude