Closes the operator-side half of the reporter's loop. The CLI fix in
the previous commit makes USER_PROJECT_DENIED errors readable to
analysts; this commit lets admins verify reachability proactively
from /admin/server-config without waiting for analyst reports.
New endpoint POST /api/admin/bigquery/test-connection
(app/api/admin_bigquery_test.py, ~110 LOC):
- Depends(require_admin); registered in app/main.py.
- Builds BqAccess via existing get_bq_access(), runs `SELECT 1 AS ok`
with a 10s polling timeout.
- 200 with {ok, billing_project, data_project, elapsed_ms} on success.
- 400 for `BqAccessError(not_configured)` (operator config issue).
- 502 for any other typed BqAccessError or unknown upstream exception.
- 504 for concurrent.futures.TimeoutError; best-effort cancel_job
invoked (BQ-side cancel may still run; documented caveat).
Server-config placeholder (app/api/admin.py + admin_server_config.html):
- `data_source.bigquery.billing_project` field-spec gains
`placeholder_from: ["data_source", "bigquery", "project"]`.
- renderLeafInput's text branch reads `opts.spec.placeholder_from`,
walks the loaded `original` config dict, injects
`placeholder="(defaults to <project>)"` into the input HTML at
construction time. Admin sees the access.py:339-340 fallback rule
visible directly in the UI without reading source.
UI button:
- "Test BigQuery connection" button next to data_source's Save button.
- onTestBigQuery() POSTs to the endpoint, renders structured result
inline (green check + elapsed_ms on success; red kind + hint on
failure).
Tests: 6 endpoint cases + 1 placeholder payload test = 7 GREEN. 62
total across the affected admin server-config test files.
