`CALL start_ui_server()` triggers extension download in DuckDB 1.5.0

After upgrading the DuckDB JDBC client to `1.5.0.0`, calling `CALL start_ui_server()` now attempts to automatically download the `ui` extension from the internet, resulting in a `SQLException`.

java.sql.SQLException: Extension Autoloading Error: An error occurred while trying to
automatically install the required extension 'ui':
Failed to download extension "ui" at URL
"http://extensions.duckdb.org/v1.5.0/osx_arm64/ui.duckdb_extension.gz"

## Environment
- DuckDB JDBC `1.5.0.0`
- macOS ARM64 (Apple Silicon)

## Steps to Reproduce
1. Add DuckDB JDBC `1.5.0.0` as a dependency in a Java project
2. Create a DuckDB connection and execute `CALL start_ui_server()`
3. Observe that DuckDB attempts to download the `ui` extension from `http://extensions.duckdb.org/v1.5.0/osx_arm64/ui.duckdb_extension.gz`
4. Call fails with `SQLException: Extension Autoloading Error`

**Note**: The `ui` extension and `start_ui_server()` were introduced in DuckDB 1.1.0. In versions prior to `1.5.0.0`, `start_ui_server()` worked without triggering an extension download from the internet.

## Expected
`CALL start_ui_server()` should not silently trigger an extension download. If the `ui` extension is not installed, it should fail with a clear error indicating the extension is missing — not attempt a
network download.

## Request
Please advise on how to invoke `start_ui_server()` without triggering an automatic extension download, or provide a way to pre-bundle the `ui` extension for offline environments.

No repository file or test is named. Start by reproducing CALL start_ui_server() through a DuckDB JDBC connection on DuckDB JDBC 1.5.0.0, then trace the ui extension's autoloading and packaging behavior; done means the call no longer silently downloads it and reports a clear offline failure or supports pre-bundling.

Scritto dal modello di indicizzazione a partire dal testo della issue.