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

[ygd58]

Closes #1313.

The response example contained a trailing comma making it invalid JSON.

// Before (invalid)
{
  "builderCode": "bc_a1b2c3d4",
  "walletAddress": "0x...",
}

// After (valid)
{
  "builderCode": "bc_a1b2c3d4",
  "walletAddress": "0x..."
}

Verified against the actual API — real response has no trailing comma.

[cb-heimdall]

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[ygd58]

@roethke @soheimam Could you please review this PR? The JSON response example in agent-builder-codes had a trailing comma making it invalid JSON. Single line fix. Thanks!