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

## Bug

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

**Current (invalid JSON):**
```json
{
"builderCode": "bc_a1b2c3d4",
"walletAddress": "0x...",
}
```

**Fix (valid JSON):**
```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