[FEATURE]: propagate W3C trace context (traceparent/baggage) in MCP tools/call _meta per SEP-414

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before. Related but different: #45997 / #46008 added _meta.sessionID (thanks, this is great); #28295 asked for traceparent as an HTTP header on remote MCP transports and was closed for inactivity. This request is about params._meta on tools/call, which is the only channel a stdio MCP server has, and follows the now-final MCP spec (SEP-414).

Describe the enhancement you want to request

What: when OpenCode invokes an MCP tool, include the W3C trace context of the active tool-call span in CallToolRequest.params._meta, using the keys the MCP spec reserves for this in SEP-414: traceparent, optionally tracestate, and baggage (unprefixed, by spec exception to the _meta prefix rule). For example:

{
  "method": "tools/call",
  "params": {
    "name": "engram_search",
    "arguments": { "...": "..." },
    "_meta": {
      "sessionID": "ses_…",
      "traceparent": "00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01",
      "baggage": "session.id=ses_…,opencode.tool.call_id=call_…"
    }
  }
}

_meta.sessionID from #46008 can stay as is; baggage just carries the same value under the spec-conventional key so servers built against the OTel GenAI MCP semantic conventions pick it up without OpenCode-specific code.

Why: OpenCode already opens an OpenTelemetry span per tool call (session/tools.ts has session.id and tool.call_id in scope at the call site; with experimental.openTelemetry those spans export). An MCP server that traces its own work today has no way to parent its spans onto that span, so the client-side and server-side halves of one tool call land in the backend as two unrelated traces. With traceparent in _meta the server span becomes a child of OpenCode's, and a trace view shows: model turn → tool call → what the server actually did (retrieval stages, cache hits, lock waits) in one waterfall. The OTel GenAI MCP semantic conventions (Development) say clients SHOULD inject propagators into params._meta and servers SHOULD use it as the parent; the Rust SDK (rmcp ≥ 3.1) already exposes RequestMetaObject::get_traceparent/get_baggage for exactly this.

Where: the same place #46008 adds _meta.sessionID (packages/core/src/mcp/client.ts), reading the current span context from the OTel API (propagation.inject into a carrier and copying the three keys) when tracing is enabled; when it is not, traceparent can simply be omitted.

Workaround today: a plugin's tool.execute.before hook can inject a reserved argument (we use args._otel = { traceparent, baggage }) and the server lifts it into _meta. It works, but it couples the server to an OpenCode-specific argument and leaks into the stored tool input; native _meta propagation makes that unnecessary.