[codex] fix flight sql explain plan rows

[bill-ph]

Summary

Fix Flight SQL EXPLAIN streaming when Duckgres advertises a synthetic one-column plan schema but DuckDB returns native explain_key / explain_value rows.

Root cause

GetQuerySchema intentionally returns a one-column physical_plan / analyzed_plan schema for EXPLAIN so schema discovery does not execute EXPLAIN ANALYZE. RowsToRecord then sized its scan destinations from that synthetic schema, while the actual DuckDB result still had two physical columns, causing sql: expected 2 destination arguments in Scan, not 1.

Changes

• Detect raw DuckDB EXPLAIN rows shaped as explain_key, explain_value when paired with the synthetic one-column plan schema.
• Scan both physical columns and emit only explain_value into the Arrow record batch.
• Add a regression test using a dedicated *sql.Conn, matching the worker session path.

Validation

• go test ./duckdbservice -run TestRowsToRecordExplainUsesPlanValueColumn -count=1
• go test ./duckdbservice -count=1
• go test ./tests/integration -run TestExplain -count=1
• just lint

[github-actions]

Test Impact Plan

Deterministic summary of how this PR changes tests, CI runners, and coverage-risk signals.

Summary

Area	Added	Changed	Deleted
Test files	0	3	0
E2E/journey files	0	2	0
Workflow files	0	0	0

Signals

• Test cases: +1 / -0
• Assertions: +18 / -0
• Skips or known failures added: 0
• Workflow continue-on-error added: 0
• Workflow path filters added: 0
• Test commands removed from justfile: 0
• E2E/journey retry lines added: 1

Coverage risk: needs review

Warnings

• E2E or journey retry behavior changed (needs review)
  • 1 retry/attempt line(s) added