Add BigQuery warehouse adapter

[carolsimone]

Add a BigQuery warehouse engine: one WarehouseAdapter implementation, published as continuo-bigquery-adapter, with a continuo-python-runtime-bigquery base image.

Part of the warehouse-engines rollout tracked in carolsimone/continuo#517. Sibling engines: Snowflake (#58), Redshift (#59).

Scope

Mirror adapters/postgres/ and adapters/trino/. A new engine is one WarehouseAdapter subclass serving both roles (data plane + validation) and one image that runs both.

• New uv workspace member adapters/bigquery/: continuo_bigquery_adapter/ (the adapter class), pyproject.toml, tests/, README.md. Add it to [tool.uv.workspace] in the root pyproject.toml.
• Implement the data-plane methods: fetch, ensure_table, load.
• Implement the validation methods: ensure_schema, drop_schema, build_empty_from_sql, build_empty_from_columns, clone_empty_from_prod, check_binds.
• Register the adapter under the continuo_engine.adapters entry-point group (so the runtime discovers it at run time).
• Publish continuo-bigquery-adapter on the shared vX.Y.Z tag (wire into publish-pypi.yml).
• Add Dockerfile.bigquery + image-requirements-bigquery.txt; wire into images.yml → base image continuo-python-runtime-bigquery (installs the pinned continuo-bigquery-adapter==X.Y.Z).
• Tests: adapter unit tests + the shared adapter conformance suite.

Engine notes

• Auth: Application Default Credentials / service account; project + GOOGLE_APPLICATION_CREDENTIALS.
• BigQuery dataset maps to the adapter's notion of schema (ensure_schema / drop_schema create/drop a dataset).
• clone_empty_from_prod: CREATE TABLE ... LIKE is not available the same way; use CREATE TABLE ... AS SELECT ... WHERE FALSE or a schema-only copy.
• Types: map the contract type set to BigQuery standard-SQL types.

Do not touch

The result-block wire contract is frozen (docs/boundary-contract.md, continuo_engine_contract.result). The Go side (pkg/validationresult in carolsimone/continuo) parses it byte-for-byte. Reuse it as-is.

Reference

• adapters/trino/ and adapters/postgres/ — the two existing engines to copy from.
• docs/boundary-contract.md — the external boundary contract.
• continuo-engine-contract (contract/) — the WarehouseAdapter port and the shared SQL/type/config guards.