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

[bayrakdarerdem]

Summary

Removes the trailing comma from the JSON response example in agent-builder-codes.mdx.

Problem

The response example contained a trailing comma which is invalid JSON and would cause errors if developers copied it directly.

Before:

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

After:

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

Fixes #1313

[cb-heimdall]

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