[CT-2969] [ModelGov3] Turn "Breaking Change to Contract" into warning, unless model is versioned

[jtcohen6]

Housekeeping

• I am a maintainer of dbt-core

Short description

• A breaking change to a model with an enforced contract should raise a warning (but CI will still succeed)
• Unless that model is versioned, in which case dbt understands that its API is locked in
• If it is versioned, you can add a new version (or just bump the existing model's version) to facilitate the breaking change
• If it's just a warning, you can still optionally promote that warning to an error (without messing with model versions) via the --warn-error / --warn-error-options config

Acceptance criteria

As the create of a model with an enforced contract, if I remove/type/rename column(s) relative to a previous invocation, I should see a warning if I perform a state comparison (state:modified).

That warning/event should have a unique name, and I should be able to promote that warning to an error via --warn-error or --warn-error-options.

As the creator of a versioned model with an enforced contract, if I remove/retype/rename column(s) relative to a previous invocation, I should still see an error if I perform a state comparison (state:modified).

Impact to Adapters

zero impact

Context

Context & conversation:

• [CT-2787] [Bug] dbt is too aggressive about preventing contracted changes during CI  #8028

dbt-core/core/dbt/contracts/graph/nodes.py

Lines 761 to 780 in 8350dfe

	# Did we find any changes that we consider breaking? If so, that's an error
	if (
	contract_enforced_disabled
	or columns_removed
	or column_type_changes
	or enforced_model_constraint_removed
	or enforced_column_constraint_removed
	or materialization_changed
	):
	raise (
	ContractBreakingChangeError(
	contract_enforced_disabled=contract_enforced_disabled,
	columns_removed=columns_removed,
	column_type_changes=column_type_changes,
	enforced_column_constraint_removed=enforced_column_constraint_removed,
	enforced_model_constraint_removed=enforced_model_constraint_removed,
	materialization_changed=materialization_changed,
	node=self,
	)
	)