feat: add BigQuery Python functions and BQClient type

Funciones CRUD completas para BigQuery: auth, datasets, tables, queries,
jobs, routines, load/export. Tipo BQClient como wrapper del SDK oficial.

python/functions/bigquery/bq_get_dataset.md

@@ -0,0 +1,45 @@
+---
+name: bq_get_dataset
+kind: function
+lang: py
+domain: infra
+version: "1.0.0"
+purity: impure
+signature: "def bq_get_dataset(client: BQClient, dataset_id: str) -> dict"
+description: "Obtiene los detalles completos de un dataset de Google BigQuery. Usa client._client.get_dataset() del SDK oficial."
+tags: [bigquery, gcp, dataset, get, google-cloud, python]
+uses_functions: []
+uses_types: []
+returns: []
+returns_optional: false
+error_type: "error_go_core"
+imports: [google-cloud-bigquery]
+params:
+  - name: client
+    desc: "instancia autenticada de BQClient"
+  - name: dataset_id
+    desc: "nombre del dataset a consultar (sin prefijo de proyecto)"
+output: "dict con dataset_id, project, full_id, location, description, labels, created, modified, default_table_expiration_ms"
+tested: false
+tests: []
+test_file_path: ""
+file_path: "python/functions/bigquery/datasets.py"
+---
+
+## Ejemplo
+
+```python
+from bigquery.client import bq_auth
+from bigquery.datasets import bq_get_dataset
+
+client = bq_auth("my-project")
+ds = bq_get_dataset(client, "analytics")
+print(ds["location"], ds["description"])
+print(ds["created"])  # ISO 8601: "2024-01-15T10:30:00+00:00"
+```
+
+## Notas
+
+Lanza `google.api_core.exceptions.NotFound` (404) si el dataset no existe.
+Los campos `created` y `modified` son strings ISO 8601 con timezone UTC, o `None` si el SDK no los retorna.
+`labels` es un dict vacio `{}` si el dataset no tiene labels.