feat(langfuse): add tool-level spans with input/output and trace-level I/O

Each MCP tool handler now sets:
- LANGFUSE_TRACE_OUTPUT_ATTR on tool spans (maps to top-level trace output)
- LANGFUSE_OBSERVATION_INPUT_ATTR and LANGFUSE_OBSERVATION_OUTPUT_ATTR
  for detailed child observation views

HTTP root span renamed to 'mcp_http_request' for consistency with stdio.
All attribute keys use constants from langfuse-transport.ts.

src/lib/langfuse-transport.ts

@@ -1,8 +1,50 @@
 import { NodeTracerProvider } from "@opentelemetry/sdk-trace-node";
 import { LangfuseSpanProcessor } from "@langfuse/otel";
 
+/** Trace name used for HTTP transport in Langfuse. */
+export const TRACE_NAME_HTTP = "ask262-http";
+
+/** Trace name used for stdio transport in Langfuse. */
+export const TRACE_NAME_STDIO = "ask262-stdio";
+
+/** Langfuse attribute key for setting the trace name on a span. */
+export const LANGFUSE_TRACE_NAME_ATTR = "langfuse.trace.name";
+
+/** Langfuse attribute key for observation input. */
+export const LANGFUSE_OBSERVATION_INPUT_ATTR = "langfuse.observation.input";
+
+/** Langfuse attribute key for observation output. */
+export const LANGFUSE_OBSERVATION_OUTPUT_ATTR = "langfuse.observation.output";
+
+/** Langfuse attribute key for trace-level input. */
+export const LANGFUSE_TRACE_INPUT_ATTR = "langfuse.trace.input";
+
+/** Langfuse attribute key for trace-level output. */
+export const LANGFUSE_TRACE_OUTPUT_ATTR = "langfuse.trace.output";
+
 let provider: NodeTracerProvider | null = null;
 
+/**
+ * Extract MCP tool call information from a JSON-RPC request body.
+ * Used to populate trace-level input in Langfuse.
+ */
+export function extractMcpToolInfo(
+  body: unknown,
+): { method?: string; tool?: string; input?: unknown } {
+  if (typeof body !== "object" || body === null) return {};
+  const b = body as Record<string, unknown>;
+  const rpcMethod = b.method as string | undefined;
+  if (rpcMethod === "tools/call") {
+    const params = b.params as Record<string, unknown> | undefined;
+    return {
+      method: rpcMethod,
+      tool: params?.name as string | undefined,
+      input: params?.arguments,
+    };
+  }
+  return { method: rpcMethod };
+}
+
 /**
  * Initialize Langfuse OTel span processor.
  * Called once at server startup before any spans are created.