Fixes #282: fix Pydantic v2 compatibility (prevent plugin startup crash due to BaseSettings import)

[DaniloVerde]

Summary

This PR fixes a startup crash in the qgis_stac plugin on QGIS LTR / Python 3.12 environments where Pydantic 2.x is installed.

Problem

When QGIS loads the plugin, classFactory fails with the following error:

pydantic.errors.PydanticImportError: BaseSettings has been moved to the pydantic-settings package.

Root Cause

In lib/planetary_computer/settings.py, the Settings class inherited from pydantic.BaseSettings.
In Pydantic 2.x, BaseSettings is no longer available from that import path.

Changes

• Added a version-tolerant import chain for BaseSettings:
  • from pydantic_settings import BaseSettings
  • fallback: from pydantic.v1 import BaseSettings
  • fallback: from pydantic import BaseSettings
• Updated the Settings class declaration to inherit from the resolved BaseSettings symbol.

Why This Approach

• Works on Pydantic 2.x without requiring immediate environment changes.
• Preserves compatibility with Pydantic 1.x and legacy setups.
• Keeps the change surface minimal (single file).

Testing and Validation

• Static check: no errors reported in the modified file.
• Runtime smoke test: loading the settings module and calling Settings.get() succeeds in local workspace validation.
• Manual verification in QGIS:
  1. Restart QGIS.
  2. Enable or reload the qgis_stac plugin.
  3. Confirm that the plugin loads without the classFactory/BaseSettings error.
• Tested on QGIS 3.44.8 - Solothurn

Impact

• Fixes plugin startup failure.
• No intended behavior change in settings handling (env file, env prefix, default values).

Notes

This is a targeted compatibility fix and does not change the plugin public API.

References

• https://docs.pydantic.dev/latest/migration/#basesettings-has-moved-to-pydantic-settings