feat(cloud): emit dataset schema metadata when a schema check runs

[m1n0]

Summary

v4 contracts / data standards now push the discovered dataset schema to Soda Cloud when a schema check runs. _build_check_collection_results_json_dict gains a top-level metadata[] block on the sodaCoreInsertScanResults payload:

{ "datasetQualifiedName": "<datasource>/<prefix…>/<dataset>",
  "schema": [ { "columnName": "id", "sourceDataType": "integer" } ] }

• Reuses the v3 discover payload shape: raw, un-parameterized data type (SqlDataType.name, e.g. character varying — not character varying(255)); isPrimaryKey omitted (the v4 schema check doesn't discover PKs; omitting preserves the backend's existing PK flags).
• datasetQualifiedName = the collection's soda_qualified_dataset_name, which resolves to the exact dataset the contract checks registered — so the backend runs its existing syncDatasetsColumns reconciliation against the right dataset.
• One entry per distinct dataset that ran a schema check; omitted entirely when no schema check discovered columns (an empty schema is never sent — the backend full-reconciles and would otherwise soft-delete every column).

Requires (end-to-end)

Backend counterpart: sodadata/soda#12702 — the contract scan handler must ingest this metadata[] block. This engine PR is inert until that lands. (Ties into DTL-1807.)

Known limitation

SqlDataType lowercases type names; the backend compares sourceDataType case-sensitively. Postgres is identical to v3. For warehouses whose catalog returns uppercase types (e.g. Snowflake TEXT/NUMBER), the first contract scan of a previously-onboarded dataset surfaces a one-time, self-converging "type changed" event. This is pre-existing shared v3/v4 comparison behavior, not introduced here.

Test plan

• soda-tests/tests/unit/test_dataset_metadata_wire.py — metadata present with the right shape; un-parameterized type; absent with no schema check; absent when the schema check found no columns; one entry per dataset in a combined multi-file session.
• Full unit suite: 918 passed, 2 skipped (no regressions in the surrounding wire tests).

DTL-1807

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud