fix(inference): update test to use current sessionId/agentId API shape

sessionId is a top-level ChatCompleteOptions field; agentId is flat on
ChatCompleteMetadata. The test was written against the original
metadata.anonymization.* shape that was refactored away by two
successive fix commits.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jonwalstedt

x-pack/platform/plugins/shared/inference/server/chat_complete/api.test.ts

@@ -207,7 +207,8 @@ describe('createChatCompleteApi', () => {
         connectorId: 'connectorId',
         system: 'original system',
         messages: [{ role: MessageRole.User, content: 'original question' }],
-        metadata: { anonymization: { sessionId: 'session-a', agentId: 'agent-a' } },
+        sessionId: 'session-a',
+        metadata: { agentId: 'agent-a' },
         maxRetries: 0,
       })
     ).resolves.toEqual(expect.objectContaining({ content: 'workflow restored' }));