[CT-2789] [Bug] Contract checksum not appropriately built for versioned models

[jtcohen6]

Versioned models do not appropriately generate their contract checksums during parsing, if the contract enforcement config is defined in yaml (as opposed to in-file {{ config }}). Meaning that same_contract checks always return True for this type of versioned model.

Repro case

Start with:

-- models/my_model.sql
select 1 as id

models:
  - name: my_model
    config:
      contract: {enforced: true}
    columns:
      - name: id
        data_type: integer

    versions:
      - v: 1

Generate a manifest and move it to a local folder:

$ mkdir state/
$ dbt parse
$ mv target/manifest.json state/

Change to:

-- models/my_model.sql
select 1 as id

models:
  - name: my_model
    config:
      contract: {enforced: true}
    columns:
      - name: id
        data_type: text

    versions:
      - v: 1

Then select modified models. I expected to see a Breaking Change to Contract Error:

$ dbt -q ls -s state:modified --state state/
09:37:41  Encountered an error:
Breaking Change to Contract Error in model my_model (models/my_model.sql)
  While comparing to previous project state, dbt detected a breaking change to an enforced contract.

  Columns with data_type changes:
   - id (integer -> text)

  Consider making an additive (non-breaking) change instead, if possible.
  Otherwise, create a new model version: https://docs.getdbt.com/docs/collaborate/govern/model-versions

Instead, it succeeds:

$ dbt -q ls -s state:modified --state state/
my_dbt_project.my_model.v1

This should raise a "Breaking Change" error that can be resolved by bumping the model version.

Potential resolutions

The problem is that a versioned model doesn't have its contract checksum appropriately generated, so this check succeeds (both are None):

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

Lines 652 to 656 in 7fbfd53

	# Otherwise: The contract was previously enforced, and we need to check for changes.
	# Happy path: The contract is still being enforced, and the checksums are identical.
	if self.contract.enforced is True and self.contract.checksum == old.contract.checksum:
	# No change -- same_contract: True
	return True

Two potential fixes:

1. Within same_contract, if not contract.checksum, generate it: 879957a
2. During parsing, call build_contract_checksum() for versioned models specifically, after configs are resolved: 434d491