Found docs updates needed from ADK python release v1.27.3 to v1.27.4

Compare link: https://github.com/google/adk-python/compare/v1.27.3...v1.27.4

### 1. **Summary of the change**

**Doc file**: docs/visual-builder/index.md

**Current state**:
> ## Next steps
>
> Using the Visual Builder development Assistant, try building a new agent using
> this prompt:

**Proposed Change**:
> ## Security and deployment
>
> The Visual Builder relies on API endpoints to save agent configuration files directly to your project directory. For security reasons, these builder endpoints are **only enabled** when running the ADK Web user interface (e.g., using `adk web`). In headless or production deployments (such as when using `adk deploy cloud_run`), the Builder endpoints are automatically disabled to prevent unauthorized file writes.
>
> Additionally, to further restrict arbitrary file writes, file uploads via the Visual Builder are strictly limited to `.yaml` and `.yml` extensions.
>
> ## Next steps
>
> Using the Visual Builder development Assistant, try building a new agent using
> this prompt:

**Reasoning**:
The FastAPI server has been updated to conditionally register the `/builder/*` endpoints only when the web UI is enabled, specifically preventing unauthorized file writes in production/headless deployments (like `cloud_run`). Additionally, uploads are now strictly limited to YAML files. These security constraints are important behaviors to document for users of the Visual Builder.

**Reference**: src/google/adk/cli/fast_api.py