fix(ui-tars): allow VLM key fields from multiple sources during preset import

[eust-w]

Summary

This PR fixes an issue where importing preset configurations would fail if any of the three VLM key fields (vlmBaseUrl, vlmApiKey, and vlmModelName) were missing, even if they were already set in the Settings-VLM settings page.

The changes allow these key fields to be provided from either source (Settings-VLM page or imported configuration), improving user experience by making the validation less strict. As long as one source provides the necessary values, the application will function correctly.

Key changes:

• Modified PresetSchema to make VLM key fields optional
• Added mergeWithExistingSettings function to combine settings from multiple sources
• Updated importPresetFromText to merge imported settings with existing ones
• Ensured Settings-VLM page is automatically updated after import

Closes: #532

Checklist

• Added or updated necessary tests (Optional).
• Updated documentation to align with changes (Optional).
• Verified no breaking changes, or prepared solutions for any occurring breaking changes (Optional).
• My change does not involve the above items.

[CLAassistant]

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