fix(dashboards): unblock filter autocomplete + drop drifty test expect

Two fixes for #2383 CI:

1. DashboardFiltersModal default-value picker no longer issues a query
   when the resolved table name is empty. A metric source with no
   metricType picked yet resolves `getMetricTableName` to
   `source.from.tableName`, which is typically empty on a metric source.
   Firing the autocomplete in that state produces a malformed
   `DESCRIBE {db}.{}` (empty Identifier substitution) that the
   ClickHouse proxy rejects with a 500. The e2e shard 2 trace shows
   exactly this 500 firing during filter editing, which left the
   dashboard filter chip dropdowns disabled long enough to time out the
   click-the-option waits. Gating on `tableName` keeps the picker idle
   until the form is configured enough to name a real table.

2. The external API HDX-4404 round-trip test asserted
   `whereLanguage: 'sql'` on the response for filters whose input payload
   omits `whereLanguage`. The Zod schema is `.optional()` with no
   default, so the wire format is undefined-in / undefined-out. The
   previous expectation made the test fail by asserting a default the
   schema never emits. Dropped the field from those three filter
   expects and from the PUT-flip filter expect; added a comment so the
   omission is intentional.

Author: alex-fedotyev

packages/api/src/routers/external-api/__tests__/dashboards.test.ts

@@ -962,13 +962,18 @@ describe('External API v2 Dashboards - old format', () => {
       // indicate schema drift) fail the assertion. The bot deep-review at
       // dashboards.test.ts:961 flagged toMatchObject for letting unexpected
       // fields slip through silently.
+      //
+      // whereLanguage is intentionally absent from these expectations: the
+      // input payload omits it, the Zod schema is optional with no default,
+      // so the round-trip is undefined-in / undefined-out. Adding the field
+      // here would make the test fail by asserting a wire-format default
+      // the schema never emits.
       expect(response.body.data.filters[0]).toEqual({
         id: response.body.data.filters[0].id,
         type: 'QUERY_EXPRESSION',
         name: 'Service (locked, read-only)',
         expression: 'ServiceName',
         sourceId: traceSource._id.toString(),
-        whereLanguage: 'sql',
         constant: true,
         renderMode: 'readonly',
       });
@@ -978,7 +983,6 @@ describe('External API v2 Dashboards - old format', () => {
         name: 'Environment (hidden)',
         expression: 'environment',
         sourceId: traceSource._id.toString(),
-        whereLanguage: 'sql',
         constant: true,
         renderMode: 'hidden',
       });
@@ -991,7 +995,6 @@ describe('External API v2 Dashboards - old format', () => {
         name: 'Region (default editable)',
         expression: 'region',
         sourceId: traceSource._id.toString(),
-        whereLanguage: 'sql',
       });
 
       // GET round-trip.
@@ -1041,7 +1044,6 @@ describe('External API v2 Dashboards - old format', () => {
         name: 'Region (now read-only)',
         expression: 'region',
         sourceId: traceSource._id.toString(),
-        whereLanguage: 'sql',
         constant: true,
         renderMode: 'readonly',
       });

packages/app/src/DashboardFiltersModal.tsx

@@ -364,8 +364,16 @@ const DashboardFilterEditForm = ({
   // default-value picker can query autocomplete values matching the
   // configured source / expression / WHERE. Memoize on the primitive
   // bits so we don't re-issue the same query on unrelated form edits.
+  //
+  // Gate on `tableName` (not just `sourceId`): a metric source with no
+  // metricType picked yet resolves to `source.from.tableName`, which is
+  // typically empty on a metric source. Firing the autocomplete in that
+  // state produces a malformed DESCRIBE ({db}.{} with an empty Identifier
+  // substitution) that the ClickHouse proxy rejects with a 500. The
+  // query only makes sense once the form has enough information to name
+  // a real table.
   const filterForValueQuery = useMemo(() => {
-    if (!sourceId || !watchedExpression) return null;
+    if (!sourceId || !watchedExpression || !tableName) return null;
     return {
       id: filter.id,
       type: 'QUERY_EXPRESSION' as const,
@@ -386,6 +394,7 @@ const DashboardFilterEditForm = ({
     watchedWhere,
     watchedWhereLanguage,
     metricType,
+    tableName,
   ]);
 
   const [modalContentRef, setModalContentRef] = useState<HTMLElement | null>(