Proposal: Add session/tools_available notification for async MCP tool readiness

[ChinaLym]

Problem

When a client creates a session via session/new with mcpServers,
MCP tool connections are established asynchronously. The client has
no way to know when tools become available. If session/prompt is
sent before MCP tools are ready, the agent operates without tools,
leading to degraded responses.

Current Behavior

1. Client sends session/new with mcpServers: [...]
2. Agent returns sessionId immediately
3. Agent begins async MCP connection (tools/list, etc.)
4. Client sends session/prompt — tool_list may be empty
5. No notification exists for tool readiness

Proposed Solution

Add an agent → client notification:

​```jsonc
{
"method": "session/tools_available",
"params": {
"sessionId": "...",
"tools": [{ "name": "...", "description": "..." }],
"source": "mcp", // "mcp" | "builtin"
"serverName": "my-server" // which MCP server (optional)
}
}

---

Alternative: extend session/update

{
"method": "session/update",
  "params": {
  "sessionId": "...",
  "kind": "tools_changed",
  "tools": [...]
  }
}

---

Use Cases

Client can show a loading indicator until tools are ready
Client can queue the first prompt and send it only after tools load
Client can display which MCP tools are available in the UI
Handles partial readiness (some servers fast, some slow)