Three client-side fixes in admin_tables.html plus a regression test
file pinning the server-side PUT contract the new JS relies on.
Bug 1 — saveBqTabEdit (synced/custom) nulled bucket/source_table on
every save; the null was supposed to clear stale state on a true
remote→materialized mode flip but fired on every save, silently
wiping persisted bucket/source_table when admin edited only the
description on an already-materialized row. Now gated by
_editOriginalQueryMode !== 'materialized'.
Bug 2/3 — _buildBigQueryPayload (synced/whole) at register time did
not send bucket/source_table — only source_query — so whole-table
materialized rows persisted with bucket=NULL. Edit modal then loaded
empty Dataset/Table inputs over a SELECT * SQL. Register now sends
both fields; _openEditBqModal additionally parses source_query as
a fallback for rows that registered pre-0.53.1.
Closes#266.
