[Bug] Client sends empty _meta:{} on every request; strict servers (Meta Ads MCP) reject with HTTP 400

从 src/mcp/shared/jsonrpc_dispatcher.py 中的 JSONRPCDispatcher.send_raw_request 开始，跟踪 out_meta 的填充方式，包括 inject_trace_context。针对严格服务器或所述的最小复现运行 initialize 请求；完成标准是：没有进度或 trace 元数据的请求完全省略 _meta，同时在存在真实元数据时保留这些元数据。

Description

JSONRPCDispatcher.send_raw_request attaches _meta to params unconditionally:

out_params = dict(params) if params is not None else {}
out_meta = dict(out_params.get("_meta") or {})
on_progress = opts.get("on_progress")
if on_progress is not None:
    out_meta["progressToken"] = request_id
out_params["_meta"] = out_meta   # <- empty dict goes on the wire

When no progress callback is set and the otel tracer is a no-op (default), out_meta stays {} and every request ships _meta: {}.

Strict JSON-RPC servers reject this: Meta's hosted Ads MCP server (https://mcp.facebook.com/ads) returns HTTP 400 with -32602 "_meta for Request must be a dict or null" on initialize, making it unreachable from any client built on this SDK. _meta: null is rejected identically — the field must be absent.

Other MCP clients Meta documents (Claude Code, ChatGPT) don't emit an empty _meta and connect fine.

Reproduction

Reported downstream from Hermes Agent (NousResearch/hermes-agent#105637), with a minimal curl repro against the real server (400 with _meta:{}}, 200 without). Happy to attach a self-contained repro against a local strict server if preferred.

Proposed fix

if out_meta:
    out_params["_meta"] = out_meta
elif "_meta" in out_params:
    del out_params["_meta"]

after inject_trace_context(out_meta) so a real trace context is still sent.

Environment

• mcp SDK: current main (src/mcp/shared/jsonrpc_dispatcher.py)
• Python 3.12