feat(bigquery): typed returns, pagination, query polling, and streaming insert for table operations

[atharva-nagane]

Closes #166

Summary

Completes the BigQuery module by converting all remaining methods to typed returns, adding the two missing operations, and documenting everything.

New response types (gcp_bigquery_types.rs):

• TableInfo { id, dataset_id } — returned by create_table
• TablePage { tables, next_page_token } — returned by list_tables
• QueryResult { rows: Vec, total_rows: Option } — returned by get_query_results
• mode: Option added to TableField for schema mode control

Method changes (gcp_bigquery.rs):

• create_table → Result<TableInfo, CloudError>
• delete_table → Result<(), CloudError>
• list_tables → Result<TablePage, CloudError> with page_token / max_results pagination
• run_query → Result<String, CloudError> (returns job ID)
• get_query_results(job_id) → Result<QueryResult, CloudError>; polls up to 60 attempts,
  sleeping 1s between each; first attempt fires immediately without sleeping
• insert_rows(dataset_id, table_id, rows) → Result<(), CloudError>; checks insertErrors
  on 200 OK and surfaces the first error message

Documentation (examples/gcp/database/bigquery.md):
Covers credentials setup, client init, create dataset/table, list with pagination,
streaming insert, query + poll loop, and delete — in CRUD order.

Tests: 25 unit tests (11 new), 0 failed.