fix(docs): remove trailing comma from JSON response example in agent-builder-codes

[ygd58]

Bug

The response example in docs/ai-agents/setup/agent-builder-codes.mdx contains a trailing comma which is invalid JSON.

Current (invalid JSON):

{
  "builderCode": "bc_a1b2c3d4",
  "walletAddress": "0x...",
}

Fix (valid JSON):

{
  "builderCode": "bc_a1b2c3d4",
  "walletAddress": "0x..."
}

Verified: The actual API response does not include a trailing comma.

Affected file: docs/ai-agents/setup/agent-builder-codes.mdx line 37