fix(langchain): load LangChainTracer synchronously to avoid span loss

[JacksonWeber]

Summary

The LangChain instrumentor was loading LangChainTracer lazily via void import("./tracer.js").then(...). Because the dynamic import resolves on a later microtask, any CallbackManager._configureSync call that fired in the window between patch() running and the import resolving silently fell through with no tracer attached:

if (instrumentor._tracerCtor) {
    args[0] = addTracerToHandlers(...);
} else {
    diag.debug("[LangChainTraceInstrumentor] LangChainTracer not yet loaded, skipping");
}

In practice the first compiled-StateGraph invoke(...) after distro startup reliably lands in that window. Its outer chain run never gets a tracer, so the outermost invoke_agent LangGraph wrapper span is never emitted, the inner agent/chat spans become trace roots, and the App Insights end-to-end view splits a single workflow run into multiple unrelated traces.

Fix

• Switch LangChainTracer from a type-only lazy import() to a static value import.
• Initialize _tracerCtor = LangChainTracer at field-declaration time so it is defined before patch() ever runs.
• Drop the if (_tracerCtor) ... else { ...skipping } branch in the wrapped _configureSync — the tracer is now guaranteed to be present.

The static import is safe here because by the time patch() runs, @langchain/core/callbacks/manager is already loaded (it is the very module argument), so transitively importing BaseTracer no longer bypasses any instrumentation hooks.

Verification

Reproduced against the public LangChain + Microsoft OTEL distro NodeJs repro at https://github.com/tokaplan/testrepo/tree/master/LangChain%20with%20Microsoft%20OTEL%20distro/NodeJs.

Before: the first protocol exercised (typically completions / AzureChatOpenAI) was missing its invoke_agent LangGraph outer span; reordering the protocols showed the missing wrapper followed whichever workflow ran first — confirming the cause was an init race, not anything client-specific.

After (built from this branch): all three protocols emit the outer invoke_agent LangGraph wrapper span on the same traceId as the inner MainWeatherAgent / chat spans, and no LangChainTracer not yet loaded, skipping debug messages are emitted.

[span] proto=completions          invoke_agent LangGraph     traceId=47edc544...
[span] proto=foundry-completions  invoke_agent LangGraph     traceId=5dbc8b58...
[span] proto=foundry-responses    invoke_agent LangGraph     traceId=f3fdc365...

npm run lint and tsc -p tsconfig.src.json --noEmit both clean.