This PR fixes an issue #7891 in the Citus planner where an `UPDATE` on a
local table with a subquery referencing a reference table could produce
a 0-task plan. Historically, the planner sometimes failed to detect that
both the target and referenced tables were effectively “local,”
assigning `INVALID_SHARD_ID `and yielding a no-op plan.
### Root Cause
- In the Citus router logic (`PlanRouterQuery`), we relied on `shardId`
to determine whether a query should be routed to a single shard.
- If `shardId == INVALID_SHARD_ID`, but we also had not marked the query
as a “local table modification,” the code path would produce zero tasks.
- Local + reference tables do not require multi-shard routing. Failing
to detect this “purely local” scenario caused Citus to incorrectly route
to zero tasks.
### Changes
**Enhanced Local Table Detection**
- Updated `IsLocalTableModification` and related checks to consider both
local and reference tables as “local” for planning, preventing the
0-task scenario.
- Expanded `ContainsOnlyLocalOrReferenceTables` to return true if there
are no fully distributed tables in the query.
**Added Regress Test**
- Introduced a new regress test (`issue_7891.sql`) which reproduces the
scenario.
- Verifies we get a valid single- or local-task plan rather than a
0-task plan.
Mehmet YILMAZ