test(ci-visibility): fix flaky agent-proxy tests

Add nock.cleanAll() to beforeEach hook to prevent HTTP mock
state leakage between tests. Without proper cleanup, nock
interceptors from previous tests could interfere with
subsequent tests, causing random failures in CI.

This follows the pattern already used in other test files
like exporter.spec.js and request.spec.js.

Fixes 4 flaky tests related to _isGzipCompatible and
evpProxyPrefix assertions.

Author: watson

packages/dd-trace/test/ci-visibility/exporters/agent-proxy/agent-proxy.spec.js

@@ -19,6 +19,7 @@ const { clearCache } = require('../../../../src/agent/info')
 describe('AgentProxyCiVisibilityExporter', () => {
   beforeEach(() => {
     clearCache()
+    nock.cleanAll()
   })
 
   const flushInterval = 50