feat(observability): propagate trace context to spawned subprocesses

[jlguerreiro]

Issue for this PR

Closes #27031

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When OpenCode spawns a tool subprocess (shell tool, stdio MCP transports, LSP servers, etc.), inject the active OTel context as TRACEPARENT (and TRACESTATE when non-empty) per the OTel env-carrier spec. Subprocess instrumentation that follows the spec then chains into OpenCode's invocation trace instead of starting a disconnected root.

How did you verify your code works?

Unit tests — added in packages/core/test/effect/observability.test.ts covering: no-op without active span, TRACEPARENT injection from a given context, sampled vs. unsampled flag encoding, invalid span
context, stale TRACEPARENT overwrite, default-arg fallback.

End-to-end check

Ran opencode with OTel enabled, prompted it to spawn an OTel-instrumented child via the shell tool, and confirmed that the child's span sits on the same trace and lists OpenCode's ShellTool.run as its parent. That parent_span_id match across process boundaries.

Script used for the E2E check

import { context, ROOT_CONTEXT, trace, TraceFlags } from "@opentelemetry/api"
import { NodeTracerProvider } from "@opentelemetry/sdk-trace-node"
import { SimpleSpanProcessor } from "@opentelemetry/sdk-trace-base"
import { OTLPTraceExporter } from "@opentelemetry/exporter-trace-otlp-http"

process.on("unhandledRejection", (err) => console.error("[child] export error:", err?.message ?? err))

function parentContextFromEnv() {
  const tp = process.env.TRACEPARENT
  if (!tp) return ROOT_CONTEXT
  const [, traceId, spanId, flags] = tp.split("-")
  return trace.setSpanContext(ROOT_CONTEXT, {
    traceId, spanId,
    traceFlags: Number.parseInt(flags, 16) || TraceFlags.NONE,
    isRemote: true,
  })
}

const provider = new NodeTracerProvider({
  spanProcessors: [new SimpleSpanProcessor(new OTLPTraceExporter())],
  const [, traceId, spanId, flags] = tp.split("-")
  return trace.setSpanContext(ROOT_CONTEXT, {
    traceId, spanId,
    traceFlags: Number.parseInt(flags, 16) || TraceFlags.NONE,
    isRemote: true,
  })
}

const provider = new NodeTracerProvider({
  spanProcessors: [new SimpleSpanProcessor(new OTLPTraceExporter())],
})
provider.register()

await context.with(parentContextFromEnv(), async () => {
  await new Promise((resolve) => {
    trace.getTracer("otel-subprocess-test").startActiveSpan("subprocess-work", (span) => {
      span.setAttribute("test.tag", "subprocess-propagation")
      setTimeout(() => { span.end(); resolve() }, 50)
    })
  })
})
try { await provider.shutdown() } catch {}

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR