feat: Add fastmcp tracing (#2695)

- Add automatic tracing for FastMCP 3.35+ and 4.x servers, including
stateless HTTP sessions.
- Record tool calls, resource reads, prompts, notifications, and errors
in Braintrust.
Use `integrations.fastmcp` to control tracing and content capture
separately from the MCP integration.
- Add `wrapFastMCP()` for manual setup:

  ```ts
  import { wrapFastMCP } from "braintrust";
  import * as fastmcp from "fastmcp";

const { FastMCP } = wrapFastMCP(fastmcp); // Add tracing to FastMCP
sessions.
  ```

Author: lforst

.changeset/add-fastmcp-tracing.md

@@ -0,0 +1,5 @@
+---
+"braintrust": minor
+---
+
+feat: Add `fastmcp` tracing

e2e/config/pr-comment-scenarios.json

@@ -626,6 +626,29 @@
       }
     ]
   },
+  {
+    "scenarioDirName": "fastmcp-instrumentation",
+    "label": "FastMCP Instrumentation",
+    "metadataScenario": "fastmcp-instrumentation",
+    "variants": [
+      {
+        "variantKey": "fastmcp-v3",
+        "label": "v3 pinned"
+      },
+      {
+        "variantKey": "fastmcp-v3-latest",
+        "label": "v3 latest"
+      },
+      {
+        "variantKey": "fastmcp-v4",
+        "label": "v4 pinned"
+      },
+      {
+        "variantKey": "fastmcp-v4-latest",
+        "label": "v4 latest"
+      }
+    ]
+  },
   {
     "scenarioDirName": "typesafe-instrumentation",
     "label": "TypeSafe Instrumentation",