feat(channels): add BlockStreamer for progressive message delivery

- Add BlockStreamer class to split streaming responses into multiple messages
- Configure block streaming with min/max chars and idle coalescing
- Integrate into ChannelBase when blockStreaming: 'on'
- Add comprehensive test coverage (16 tests)

This improves UX by delivering completed paragraphs as separate messages
instead of waiting for the full response.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

packages/channels/base/tsconfig.json

@@ -5,5 +5,5 @@
     "rootDir": "src"
   },
   "include": ["src/**/*.ts"],
-  "exclude": ["node_modules", "dist"]
+  "exclude": ["node_modules", "dist", "src/**/*.test.ts"]
 }