fix(spanner): process metadata and stats for PartialResultSets with empty values

[apstndb]

Problem

RowIterator::try_recv() returns early when result_set.values.is_empty(), skipping all subsequent processing — metadata, resume_token, and stats are silently discarded.

For reference, the Go client's RowIterator documents the expected availability of these fields:

// The plan for the query. Available after RowIterator.Next returns
// iterator.Done if QueryWithStats was called.
QueryPlan *sppb.QueryPlan

// Execution statistics for the query. Available after RowIterator.Next
// returns iterator.Done if QueryWithStats was called.
QueryStats map[string]interface{}

// The metadata of the results of the query. The metadata are available
// after the first call to RowIterator.Next(), unless the first call to
// RowIterator.Next() returned an error that is not equal to iterator.Done.
Metadata *sppb.ResultSetMetadata

The Rust RowIterator provides equivalent accessors — columns_metadata(), stats() — but they were broken for empty result sets because try_recv() discarded the underlying data before it could be stored.

Affected scenarios

When values are empty (either because the mode returns no rows, or because the query matches 0 rows), all information in the PartialResultSet was discarded. The following table shows what each mode is expected to return and what was lost:

QueryMode	When affected	Lost information
Plan	Always (returns no values)	columns_metadata(), stats().query_plan
Profile	0 result rows	columns_metadata(), stats().query_plan, stats().query_stats
WithPlanAndStats	0 result rows	columns_metadata(), stats().query_plan, stats().query_stats
WithStats	0 result rows	columns_metadata(), stats().query_stats
Normal	0 result rows	columns_metadata()

Fix

Move the values.is_empty() check after resume_token and stats processing, and call self.rs.add() before returning Ok(false) so that metadata is always captured.

Tests

Added three integration tests in spanner/tests/transaction_ro_test.rs:

• test_query_plan_mode_metadata — QueryMode::Plan populates columns_metadata() despite returning no data rows. On real Spanner, Plan mode also returns stats() with query_plan (plan_nodes), but the emulator does not, so query_plan is not asserted here.
• test_query_empty_result_metadata — QueryMode::Normal with WHERE FALSE still populates columns_metadata().
• test_query_profile_mode_empty_result_stats — QueryMode::Profile with 0 rows populates both columns_metadata() and stats() (query_stats).

All three tests fail without the fix and pass with it. Existing tests pass with no regressions. Additionally, verified against a real Spanner instance that QueryMode::Plan returns stats().query_plan with populated plan_nodes.