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

[schoolkamsergj]

Closes #1313

What changed? Why?
The JSON response example in docs/ai-agents/setup/agent-builder-codes.mdx contained a trailing comma after "walletAddress", making it invalid JSON.

Trailing commas are not valid in JSON (RFC 8259) and will cause parse errors in any strict JSON parser.

Change

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

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

How has it been tested?
Validated with JSON.parse() — no errors after the fix.

[cb-heimdall]

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