Automatically use active CMakePreset

### Brief Issue Summary

Assume that
1. a `CMakePresets.json` is present and contains a single preset
2. `cmake.buildDirectory` points to a build directory that is already configured for a preset

then the cmake extension still shows a drop down selection when first opening the project.
Instead the extension could automatically use this preset.

Use case: I want to minimize configuration efforts when the workspace is first opened.
If there is any other possibility to specify a default preset, this would also work for me.

### CMake Tools Diagnostics

_No response_

### Debug Log

_No response_

### Additional Information

_No response_

Start by tracing how CMakePresets.json and the cmake.buildDirectory setting are handled when a workspace first opens, especially where the preset-selection dropdown is shown. Reproduce the single-preset case and verify that the existing configured preset is selected automatically without prompting.

Written by the indexing model from the issue text.