Chore!: Make default_connection optional

[VaggelisD]

Up until now the config's default_connection was default initialized to DuckDB for less setup friction.

However, if the gateway connection was not configured properly, this could lead to us silently replacing it with DuckDB.

Our tests largely depended on that fact so as to reduce boilerplate code, and by making it optional there were many failures introduced. For that matter, I went ahead and solved it by monkey-patching Config.get_connection().

[Copilot]

Pull Request Overview

Makes the default_connection field optional and enforces explicit configuration by raising ConfigError when no connection is provided, and updates tests to cover this behavior.

• Change default_connection default to None and update get_connection to error when unset
• Add new integration tests for missing connection configuration
• Monkey-patch Config.get_connection in test setup for lax fallback

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
sqlmesh/core/config/root.py	Made default_connection optional and added error on missing connection in get_connection
tests/init.py	Introduced original_get_connection and a lax override for testing, patching Config.get_connection
tests/core/test_integration.py	Added test_missing_connection_config covering new error paths and importing ConfigError

Comments suppressed due to low confidence (3)

tests/core/test_integration.py:6138

• The code uses mock.patch.object but mock is not imported in this file; add from unittest import mock or import mock at the top to avoid a NameError.

with mock.patch.object(Config, "get_connection", Config.original_get_connection):

tests/init.py:20

• Avoid using a bare except: which catches all exceptions; catch specific exceptions (e.g., except ConfigError:) to prevent masking unrelated errors.

    except:

tests/init.py:12

• [nitpick] The comment mentions Config._get_connection but the override actually replaces Config.get_connection; update the comment to reflect the correct method name.

# Replace the implementation of Config._get_connection with a new one that always