Derive the dbt contract id from the manifest's project_name

[DMZ22]

Closes #1221.

import_dbt_manifest called create_odcs() with no arguments, so odcs_helper.create_odcs's id=id or "my-data-contract" default applied to every dbt import. The project name was already being read one line later for odcs.name, so the information was there — just not used for the id.

$ datacontract import --format dbt --source manifest_jaffle_duckdb.json
  before:  id: my-data-contract    name: jaffle_shop
  after:   id: jaffle_shop         name: jaffle_shop

I normalise the id with .lower().replace(" ", "-"), matching powerbi_importer.py, which is the only other importer that sets an id today. dbt project names are already snake_case by dbt's own naming rules, so this is a no-op for real manifests and only matters for hand-edited ones — happy to drop it and assign project_name verbatim if you'd rather keep it literal.

A manifest with no project_name still falls back to my-data-contract, so nothing changes for malformed input.

Tests. The four expected fixtures all asserted id: my-data-contract, so they are updated to jaffle_shop. Three new tests cover the behaviour directly: the id comes from the project name (checked against both jaffle_shop and test_project), it is slugified for a name with spaces and capitals, and it still falls back when project_name is absent.

Reverting only dbt_importer.py while keeping the tests takes tests/test_import_dbt.py from 15 passed to 6 failed, so the new tests do guard the change rather than just restate it.

No regressions. pytest -k import --continue-on-collection-errors gives 24 failed / 34 errors both before and after — all of them optional backends (S3, Trino, SQL Server) that need extras I don't have installed locally — while passes go 73 → 76, which is exactly the three new tests.

• Tests pass (uv run pytest)
• Code formatted (uv run ruff check --fix && uv run ruff format)
• Docs updated (if relevant) — docs/docs/imports/dbt.md shows no sample contract, so nothing to change
• CHANGELOG.md entry added