[CT-2401] [Bug] Adding a new column to a model with an enforced contract should not be a breaking change

[jtcohen6]

Reported in #7065 (comment)

Reproduction case

-- models/sometable.sql
select 1 as id

models:
  - name: sometable
    config:
      contract:
        enforced: true
    columns:
      - name: order_id
        data_type: number
      - name: order_name
        data_type: varchar

Run:

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

Change the yaml to:

models:
  - name: sometable
    config:
      contract:
        enforced: true
    columns:
      - name: order_id
        data_type: number
      - name: order_name
        data_type: varchar
      - name: categories
        data_type: varchar

$ dbt ls -s state:modified --state state/
10:38:49  Running with dbt=1.5.0-b5
10:38:50  Found 2 models, 0 tests, 0 snapshots, 1 analysis, 536 macros, 0 operations, 1 seed file, 0 sources, 0 exposures, 0 metrics, 0 groups
10:38:50  Encountered an error:
Contract Error in model sometable (models/sometable.sql)
  There is a breaking change in the model contract because column definitions have changed; you may need to create a new version. See: https://docs.getdbt.com/docs/collaborate/publish/model-versions

Also

Let's rename ModelContractError to ModelContractBreakingChangeError, to better disambiguate it from ContractError (when the model's contract fails at build time).