fix: ignore local config during tests (#3006)

# Rationale for this change

Fixes a test failure that hits anyone with PyIceberg already configured.

The test uses `load_catalog("default", type="in-memory")`, which merges
your dev environment config with the test parameters. If you have a
`default` catalog defined in `~/.pyiceberg.yaml` with something like
uri: https://best-rest-catalog.com, the merge produces `{"uri":
"https://best-rest-catalog.com", "type": "in-memory"}`. SQLAlchemy sees
that https:// URI and tries to load it as a database dialect, which
fails wuth:

```
sqlalchemy.exc.NoSuchModuleError: Can't load plugin: sqlalchemy.dialects:https
```

Therefore, this PR avoids using the local pyicberg.yaml file entirely!

## Are these changes tested?

Yes. I created a conflicting ~/.pyiceberg.yaml with a default catalog
pointing to an HTTPS REST endpoint, confirmed the old code fails, and
verified the fix bypasses the config merge and works.

## Are there any user-facing changes?

No

Author: geruh

tests/conftest.py

@@ -40,6 +40,7 @@
     TYPE_CHECKING,
     Any,
 )
+from unittest import mock
 
 import boto3
 import pytest
@@ -112,6 +113,20 @@ def pytest_collection_modifyitems(items: list[pytest.Item]) -> None:
             item.add_marker("unmarked")
 
 
+@pytest.fixture(autouse=True, scope="session")
+def _isolate_pyiceberg_config() -> None:
+    """Make test runs ignore your local PyIceberg config.
+
+    Without this, tests will attempt to resolve a local ~/.pyiceberg.yaml while running pytest.
+    This replaces the global catalog config once at session start with an env-only config.
+    """
+    import pyiceberg.catalog as _catalog_module
+    from pyiceberg.utils.config import Config
+
+    with mock.patch.object(Config, "_from_configuration_files", return_value=None):
+        _catalog_module._ENV_CONFIG = Config()
+
+
 def pytest_addoption(parser: pytest.Parser) -> None:
     # S3 options
     parser.addoption(