fix: avoid loading dotenv during import

[mikemikimike]

Closes #3666

Importing pymilvus currently calls load_dotenv() from settings.py, causing every variable in a caller's working-directory .env file to be injected into the process environment. This is an unexpected library import side effect.

This change removes the import-time dotenv loading while preserving the existing environment-variable configuration reads. A subprocess regression test verifies that a .env value is not injected when pymilvus.settings is imported.

Validation: focused settings and utility tests passed (71); Ruff format/check and git diff --check passed.

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[mikemikimike]

I rechecked this PR for issue #3666. The PR is linked to the issue, and there is no human changes-requested review or failing check reported for the current head. Could a maintainer please review whether the change fully resolves the issue and approve it, or advise on any remaining work?